Como Gandalf sabia que Nazgûl atravessava o rio em segredo?

Saruman disse a Radagast.

’ ”I have an urgent errand,” he said. “My news is evil.” Then he looked about him, as if the hedges might have ears. “Nazgûl,” he whispered. “The Nine are abroad again. They have crossed the River secretly and are moving westward. They have taken the guise of riders in black.”

‘I knew then what I had dreaded without knowing it

’ “The Enemy must have some great need or purpose,” said Radagast; “but what it is that makes him look to these distant and desolate parts, I cannot guess.”

’ “What do you mean?” said I.

’ ”I have been told that wherever they go the Riders ask for news of a land called Shire.”

' “The Shire,” I said; but my heart sank. For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain. A great king and sorcerer he was of old, and now he wields a deadly fear. “Who told you, and who sent you?” I asked.

‘“Saruman the White,” answered Radagast. “And he told me to say that if you feel the need, he will help; but you must seek his aid at once, or it will be too late.”

(LotR, Conselho de Elrond)

Como Saruman sabia que não nos disseram, mas ele tinha o palantir e estava ligado a Sauron. De fato, os Espectros do Anel receberam a ordem de parar por Isenguard e se encontraram com Saruman, embora isso tenha acontecido depois que Saruman enviou essa mensagem.

Supondo que ele não soubesse através desses canais, ele pode ter ouvido rumores de um grande terror e ligado os pontos.

The Lord of Morgul therefore led his companions over Anduin, unclad and unmounted, and invisible to eyes, and yet a terror to all living things that they passed near. It was, maybe, on the first day of July that they went forth. They passed slowly and in stealth, through Anŏrien, and over the Entwade, and so into the Wold, and rumour of darkness and a dread of men knew not what went before them. They reached the west-shores of Anduin a little north of Sarn Gebir, as they had trysted; and there received horses and raiment that were secretly ferried over the River. This was (it is thought) about the seventeenth of July. Then they passed northward seeking for the Shire, the land of the Halflings.

About the twenty-second of July they met their companions, the Nazgŭl of Dol Guldur, in the Field of Celebrant. There they learned that Gollum had eluded both the Orcs that recaptured him, and the Elves that pursued them, and had vanished.2 They were told also by Khamŭl that no dwelling of Halflings could be discovered in the Vales of Anduin, and that the villages of the Stoors by the Gladden had long been deserted. But the Lord of Morgul, seeing no better counsel, determined still to seek northward, hoping maybe to come upon Gollum as well as to discover the Shire. That this would prove to be not far from the hated land of Lŏrien seemed to him not unlikely, if it was not indeed within the fences of Galadriel. But the power of the White Ring he would not defy, nor enter yet into Lorien. Passing therefore between Lŏrien and the Mountains the Nine rode ever on into the North; and terror went before them and lingered behind them; but they did not find what they sought nor learn any news that availed them.

(Contos Inacabados, Caça ao Anel)

No Senhor do Anel: Companheiro de um leitor, há ainda essa nota que eles citam Tolkien tendo feito, abordando alguns problemas de cronologia - essencialmente, o conhecimento de Radagast de que os Espectros do Anel cruzaram o rio é "muito rápido" na narrativa. Isso sugere uma travessia anterior, antes do ataque a Osgiliath:

The Nazgul are ordered to steal over Anduin one by one and make enquiries. This is ordered soon after S[auron] learns that Gollum (who disappeared into the D[ead] Marshes) has been captured and is with Thranduil, and that Gandalf has visited that realm. Say sometime early in April. [...] At first the Nazgul investigate Anduin's Vale ... but can find no trace of Ring or 'Baggins' ... some begin to investigate Rohan.... Sauron is already in communication with Saruman [illegible] Palantir; but has not yet mastered him. Yet he reads enough of his mind to suspect (a) that he covets the Ring for himself and (b) that he knows something about it. The Nazgul are ordered to visit Saruman. Saruman is v[ery] frightened at S[auron]'s suspicion of himself and his knowledge of the Ring. Though he dislikes Gandalf intensely and is v[ery] jealous of him, he believes G[andalf] knows something vital about the Ring because the Nazgul [?ask] of the Shire which has always been a great concern of Gandalf, and because his agents have discovered that it is extraord[i-narily] closely guarded; also that Gandalf is now actually there since 12 April. He [Saruman] therefore thinks of getting his [Gandalf's] help... . The Nazgul, then, came to Isengard towards early? June. Saruman was helped at this point [by] g[ood] fortune. Radagast becomes aware that Nazgul are abroad in Anduin Vale spreading panic and searching for 'Shire'. He becomes v[ery] alarmed and can think of nothing but to go and consult Saruman head of order of Wizards. He does so ... not long after visit of the Nazgul to Isengard. Saruman knows that Radagast is a kinsman of Gandalf's and wholly trusted by him: he uses him as messenger, and sends him off to the Shire. Radagast leaves Isengard about June 15th. ...

Isso nos sugere, de qualquer maneira, que tanto Radagast (dos relatos dos terrores que estão sendo espalhados) quanto Saruman (através da comunicação com Sauron) sabiam dos Espectros do Anel que viriam para o Ocidente.

O que sabemos do LotR que pode ter sobre a questão da fonte da informação?

Graças a @Harry Johnson nos comentários, parece provável que Saruman tenha descoberto que os Cavaleiros estavam no exterior. Como Gandalf narra mais tarde

Radagast:

"I have been told that wherever they go the Riders ask for news of a land called Shire."

Gandalf:

' "The Shire," I said; but my heart sank. For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain. A great king and sorcerer he was of old, and now he wields a deadly fear. "Who told you, and who sent you? " I asked.

Radagast:

' "Saruman the White," answered Radagast. "And he told me to say that if you feel the need, he will help; but you must seek his aid at once, or it will be too late."

Isso parece claro, mas ainda é insatisfatório, porque deixa o quebra-cabeça de como Saruman tinha Radagast para usar como mensageiro, já que tudo o que sabemos aponta para Radagast como bastante envolvido e certamente não morando em Isengard. Não acho credível que Saruman soubesse encontrar Radagast e o chamasse - um orc, talvez? - para usar como um mensageiro. Se ele tinha um mensageiro para chamar Radagast, ele tinha um mensageiro para enviar a Gandalf - e o tempo era essencial.

A melhor explicação para aquele Radagast encontrou algum rastro dos Cavaleiros que o preocuparam o suficiente para consultar Saurman, que o descobriu independentemente ou, alertado por Radagast, descobriu a verdadeira natureza dos Cavaleiros e, com Radagast à mão, o enviou para avisar Gandalf.

(Observe que ele montou um cavalo, não alguns %Coelhos!

I came upon a traveller sitting on a bank beside the road with his grazing horse beside him. It was Radagast the Brown

fim da digressão.)

Radagast poderia ter descoberto os Cavaleiros? Gandalf diz:

Radagast is, of course, a worthy Wizard, a master of shapes and changes of hue; and he has much lore of herbs and beasts, and birds are especially his friends.

Especialmente considerando a frequência com que nas obras de Tolkien as pessoas (boas ou más) usam pássaros para espiões, Radagast pode muito bem estar de olho nas coisas que usam pássaros. Quando eles avistaram os Cavaleiros Negros, mesmo que ele não soubesse o que eles eram, além de serem maus, seu impulso natural seria ir a Saruman em busca de conselhos.

Como Saruman poderia tê-los descoberto? Talvez usando o Crebain ele parecia controlar.

'Regiments of black crows are flying over all the land between the Mountains and the Greyflood,' he said, 'and they have passed over Hollin. They are not natives here; they are crebain out of Fangorn and Dunland. I do not know what they are about: possibly there is some trouble away south from which they are fleeing; but I think they are spying out the land.'

Independentemente disso, não há dúvida de que Saruman enviou Radagast em sua missão para encontrar Gandalf, mas isso não significa que ele seja a fonte original da informação.

Quem primeiro descobriu que os Cavaleiros estavam no exterior, agora Saruman sabia. E Saruman suspeitava que Gandalf sabia alguma coisa sobre o Anel e suspeitava de uma conexão com o Condado:

That is in truth why I brought you here. For I have many eyes in my service, and I believe that you know where this precious thing now lies. Is it not so? Or why do the Nine ask for the Shire, and what is your business there?"

Então, por que ele alertou Gandalf? Saruman ainda não era um tolo total, e percebeu que impedir Sauron de recuperar o Anel era ainda mais essencial do que adquiri-lo. Então: Avise Gandalf, salve o Anel de Sauron e talvez pegue pistas vitais sobre o que Gandalf sabia.

Então: pegue o anel; Derrote Rohan; Derrote Gondor, Suborn ou derrote Gandalf; Derrote Sauron. Todos apenas detalhes de implementação.

E no que diz respeito à resposta à pergunta específica: Gandalf sabia porque Radagast havia lhe dito. Radagast aprendeu informações críticas com Saruman, mas pode ter descoberto parte do problema por conta própria.