A aparência do One Ring está descrita nos livros?

O Um Anel é um anel simples, dourado e sem adornos. Aqui está a descrição de Shadow of the Past :

Gandalf held it up. It looked to be made of pure and solid gold. 'Can you see any markings on it?' he asked.
'No,' said Frodo. 'There are none. It is quite plain, and it never shows a scratch or sign of wear.'

E aqui está descrito novamente, desta vez nas palavras de Saruman, do Conselho de Elrond :

"The Nine, the Seven, and the Three," he said, "had each their proper gem. Not so the One. It was round and unadorned, as it were one of the lesser rings; but its maker set marks upon it that the skilled, maybe, could still see and read."

E aqui está a descrição de Isildur, novamente do Conselho de Elrond :

Yet even as I write it is cooled, and it seemeth to shrink, though it loseth neither its beauty nor its shape. Already the writing upon it, which at first was as clear as red flame, fadeth and is now only barely to be read. It is fashioned in an elven-script of Eregion, for they have no letters in Mordor for such subtle work; but the language is unknown to me. I deem it to be a tongue of the Black Land, since it is foul and uncouth.

Esta escrita, claro, foi a Inscrição do Anel.

De A Sociedade do Anel (livro), esta passagem pelo menos menciona a cor.

As for Bilbo Baggins, even while he was making his speech, he had been fingering the golden ring in his pocket: his magic ring that he had kept secret for so many years.

Assim como no prólogo, descrevendo Gollum:

He possessed a secret treasure that had come to him long ages ago, when he still lived in the light: a ring of gold that made its wearer invisible.

Também em Wikipédia:

The words of the ring-inscription are in Black Speech, a language devised by Sauron and used in the land of Mordor. The inscription reflects the One Ring's power to control the other Rings of Power. The writing uses Elvish letters (tengwar), in a mode adapted to the Black Speech.

The Ring had the ability to change size. As well as adapting to fingers of varying size, from Sauron's to Frodo's, it sometimes suddenly expanded in order to give its wearer the slip

O tamanho e a forma não são tão concretos quanto sua cor.

Normally the One Ring appeared perfectly plain and featureless, but when heated its inscription appeared in fiery letters. A drawing of the inscription and a translation provided by Gandalf appears in Book I, Chapter 2 of The Fellowship of the Ring, "The Shadow of the Past".

Gandalf fala as palavras em Black Speech no Livro II, capítulo 2, "The Council of Elrond":

"Ash nazg durbatulûk, ash nazg gimbatul,

Ash nazg thrakatulûk agh burzum-ishi krimpatul."

Traduzido, as palavras significam:

One ring to rule them all, one ring to find them,

One ring to bring them all and in the darkness bind them.

O Anel era um círculo perfeito de ouro puro. Os outros anéis de poder tinham pedras embutidas neles, mas o Um Anel era apenas uma faixa simples - exceto quando aquecidos, as linhas finas de fogo revelavam o roteiro Tengwar da Língua Negra.

O tamanho do anel é um pouco duvidoso, parece ser capaz de se expandir e contrair para se adaptar ao usuário - sejam do tamanho de Isildur ou de Smeagol.