Como Frodo sabia onde ficava a vila de Bree?

Essa é uma das diferenças entre os livros e os filmes.

Os livros

Nos livros, Gandalf simplesmente diz a Frodo para ir a Rivendell. Ele nunca menciona Bree.

‘No indeed!’ said Frodo. ‘But in the meantime what course am I to take?’

‘Towards danger; but not too rashly, nor too straight,’ answered the wizard. ‘If you want my advice, make for Rivendell. That journey should not prove too perilous, though the Road is less easy than it was, and it will grow worse as the year fails.’

‘Rivendell!’ said Frodo. ‘Very good: I will go east, and I will make for Rivendell.

_{The Lord of the Rings Book One, Chapter 2: The Shadow of the Past
Page 66 (Houghton Mifflin Harcourt; Single Volume 50th Anniversary Edition)}

Frodo não esteve em Rivendell, mas ele sabe disso pelas histórias de Bilbo sobre sua grande jornada. Ele saberá que, se seguir a Great Road a leste do Condado, alcançará Rivendell.

Ele provavelmente também sabe que passará pela vila de Bree a caminho de Rivendell. Os hobbits do Condado conhecem Bree; o Condado foi fundado por hobbits que viajavam de Bree.

About this time legend among the Hobbits first becomes history with a reckoning of years. For it was in the one thousand six hundred and first year of the Third Age that the Fallohide brothers, Marcho and Blanco, set out from Bree; and having obtained permission from the high king at Fornost, they crossed the brown river Baranduin with a great following of Hobbits. They passed over the Bridge of Stonebows, that had been built in the days of the power of the North Kingdom, and they took all the land beyond to dwell in, between the river and the Far Downs.

_{The Lord of the Rings Prologue, Section 1: Concerning Hobbits
Page 4 (Houghton Mifflin Harcourt; Single Volume 50th Anniversary Edition)}

Embora isso tenha acontecido há cerca de 10 anos, ainda existem alguns hobbits da Shire (principalmente do leste de Farthing e Buckland) que visitam Bree. Quando Frodo e seus companheiros estão se aproximando de Bree, Merry diz a eles

‘There are hobbits in Bree,’ said Merry, ‘as well as Big Folk. I daresay it will be homelike enough. The Pony is a good inn by all accounts. My people ride out there now and again.’

_{The Lord of the Rings Book One, Chapter 8: Fog on the Barrow-downs
Page 148 (Houghton Mifflin Harcourt; Single Volume 50th Anniversary Edition)}

Os filmes

Gandalf saberia que Frodo está ciente da existência de Bree e que ele pelo menos sabe que fica a leste do Condado. Existe apenas uma estrada principal que leva a leste a partir do Condado, então Frodo não deve ter problemas para encontrá-la.

Se Frodo não tivesse certeza do caminho, ele provavelmente pediria instruções a Gandalf. Como ele não sabe, podemos assumir que ele sabia como chegar lá.

No livro, Tom Bombadil diz a eles, embora eles certamente já conhecessem Bree de antemão.

All the same the hobbits wished he was coming with them. They felt that he would know how to deal with Black Riders, if anyone did. They would soon now be going forward into lands wholly strange to them, and beyond all but the most vague and distant legends of the Shire, and in the gathering twilight they longed for home. A deep loneliness and sense of loss was on them. They stood silent, reluctant to make the final parting, and only slowly became aware that Tom was wishing them farewell, and telling them to have good heart and to ride on till dark without halting.

Tom will give you good advice, till this day is over (after that your own luck must go with you and guide you): four miles along the Road you'll come upon a village, Bree under Bree-hill, with doors looking westward. There you'll find an old inn that is called The Prancing Pony. Barliman Butterbur is the worthy keeper. (Fog on the Barrow-downs)

O 'povo' de Merry visitava de tempos em tempos.

‘I am sorry to take leave of Master Bombadil,’ said Sam. ‘He's a caution and no mistake. I reckon we may go a good deal further and see naught better, nor queerer. But I won't deny I'll be glad to see this Prancing Pony he spoke of. I hope it'll be like The Green Dragon away back-home! What sort of folk are they in Bree?’

There are hobbits in Bree,’ said Merry, ‘as well as Big Folk. I daresay it will be homelike enough. The Pony is a good inn by all accounts. My people ride out there now and again.’ (Fog on the Barrow-downs)

Bree-land era a única área habitada na região:

Bree was the chief village of the Bree-land, a small inhabited region, like an island in the empty lands round about Besides Bree itself, there was Staddle on the other side of the hill, Combe in a deep valley a little further eastward, and Archet on the edge of the Chetwood. Lying round Bree-hill and the villages was a small country of fields and tamed woodland only a few miles broad. (At the Sign of the Prancing Pony)

e Hobbits, da Bree-lander, alegavam ser anteriores ao Condado, e sabiam-se que tinham relações mútuas, principalmente com Eastfarthing, a apenas um dia de viagem.

There were also many families of hobbits in the Bree-land and they claimed to be the oldest settlement of Hobbits in the world, one that was founded long before even the Brandywine was crossed and the Shire colonized. [...] The Bree-folk, Big and Little, did not themselves travel much; and the affairs of the four villages were their chief concern. Occasionally the Hobbits of Bree went as far as Buckland, or the Eastfarthing; but though their little land was not much further than a day's riding east of the Brandywine Bridge, the Hobbits of the Shire now seldom visited it. An occasional Bucklander or adventurous Took would come out to the Inn for a night or two, but even that was becoming less and less usual. [...] It was not yet forgotten that there had been a time when there was much coming and going between the Shire and Bree. There was Bree-blood in the Brandybucks by all accounts.(The Sign of the Prancing Pony)

Do segundo capítulo, "A Sombra do Passado" (grifo meu):

He lived alone, as Bilbo had done; but he had a good many friends [...] Frodo went tramping all over the Shire with them; but more often he wandered by himself, and to the amazement of sensible folk he was sometimes seen far from home walking in the hills and woods under the starlight. Merry and Pippin suspected that he visited the Elves at times, as Bilbo had done.

Então aprendemos que Frodo viajou muito, com seus amigos ou até sozinho: ele claramente conhece muito bem o lugar. Além disso, ele fala com os elfos (mais sobre isso mais tarde).

[...]

Frodo himself, after the first shock, found that being his own master and the Mr. Baggins of Bag End was rather pleasant. For some years he was quite happy and did not worry much about the future. But half unknown to himself the regret that he had not gone with Bilbo was steadily growing. He found himself wondering at times, especially in the autumn, about the wild lands, and strange visions of mountains that he had never seen came into his dreams. He began to say to himself: 'Perhaps I shall cross the River myself one day.' To which the other half of his mind always replied: 'Not yet.'

Ele começa a pensar nas "terras selvagens" (fora do Condado), e está dividido entre sair e ficar em casa; e mesmo que ele escolha não sair, ele ainda pretende fazê-lo posteriormente. Ainda mais: ele não pensa genericamente em "partir", mas especificamente em "atravessar o rio" (o Brandywine / Baranduin), que fica no lado leste do Condado, em direção a Bree. Assim, mesmo a direção que ele tem em mente é a que leva a Bree. Afinal, é a mesma direção que Bilbo tomou, o que significa muito para ele, pois lamenta não ter ido embora.

So it went on, until his forties were running out, and his fiftieth birthday was drawing near: fifty was a number that he felt was somehow significant (or ominous); it was at any rate at that age that adventure had suddenly befallen Bilbo. Frodo began to feel restless, and the old paths seemed too well-trodden. He looked at maps, and wondered what lay beyond their edges: maps made in the Shire showed mostly white spaces beyond its borders. He took to wandering further afield and more often by himself; and Merry and his other friends watched him anxiously. Often he was seen walking and talking with the strange wayfarers that began at this time to appear in the Shire.

Aqui vemos que Frodo está ficando cansado de ficar no Condado, e ele consulta mapas. E, embora seja verdade que "os mapas feitos no Condado mostravam principalmente espaços em branco além de suas fronteiras", é seguro assumir que Bree, sendo a única cidade a leste do Condado¹, onde vivem vários hobbits, é um lugar conhecido, relatado por todos os mapas. Bree não é aquele longe depois de tudo; de acordo com o mapa que acompanha o livro, da ponte de Brandywine a Bree, é um pouco mais do que o quilômetro 100; conforme relatado por Resposta de Shamshiel, "[Bree] não estava muito além de um dia andando a leste da ponte Brandywine"). Então se um lugar fora do Condado é conhecido, é Bree.

There were rumours of strange things happening in the world outside; and as Gandalf had not at that time appeared or sent any message for several years, Frodo gathered all the news he could. Elves, who seldom walked in the Shire, could now be seen passing westward through the woods in the evening, passing and not returning; but they were leaving Middle-earth and were no longer concerned with its troubles. There were, however, dwarves on the road in unusual numbers. The ancient East-West Road ran through the Shire to its end at the Grey Havens, and dwarves had always used it on their way to their mines in the Blue Mountains. They were the hobbits' chief source of news from distant parts - if they wanted any: as a rule dwarves said little and hobbits asked no more. But now Frodo often met strange dwarves of far countries, seeking refuge in the West. They were troubled, and some spoke in whispers of the Enemy and of the Land of Mordor.

Aqui vemos que Frodo conversa com "estranhos viajantes" (na verdade mencionados no parágrafo anterior), incluindo Elfos (como Merry e Pippin suspeitavam no primeiro parágrafo que citei) e Anões. Todos esses viajantes vêm do leste (os elfos estão "passando para o oeste" e os anões estão "buscando refúgio no oeste"), então é natural que parem em Bree, mesmo que apenas por uma noite, e que disse Frodo sobre isso.

Conclusão

Frodo costumava viajar muito, estava muito interessado no que estava fora do Condado, queria ir para o leste (seguindo Bilbo), estudava mapas, conversava com vários viajantes vindos do leste e Bree é praticamente o único lugar ao leste do Condado¹. Embora nunca tivesse estado em Bree antes, certamente sabia muito bem onde estava.

¹ Na verdade, havia também Staddle, Combe e Archet, mas são menores e estão por perto, a ponto de serem conhecidos coletivamente como "Bree-land".