The book was originally illustrated by Pauline Baynes and later by Roger Garland.
The book, like the first edition of The Fellowship of the Ring, is presented as if it is an actual translation from the Red Book of Westmarch, and contains some background information on the world of Middle-earth which is not found elsewhere. Examples are the name of the tower at Dol Amroth and the names of the Seven Rivers of Gondor. There is some dispute about its canonical status since the information presented about the secondary world is considered only as folklore among the Hobbits.
It is also notable because it uses the letter 'K' instead of 'C' for the /k/ sound in Sindarin, a spelling variant Tolkien alternated many times in his writings.
Tom Bombadil can best be seen as a small, poetic venture into Tolkien's imagination.
It is clear, though, that Bombadil was not in Tolkien's conception part of Middle-earth from the start; he was invented in honour of a Dutch doll belonging to his children, to whom Tolkien told stories about TomBombadil.
TomBombadil was, however, part of The Lord of the Rings from the earliest drafts.
Bombadil could be part of the Music of the Ainur and that would explain why he was there in the beginning, but if he was indeed part of the music, it is not said why he exists.
That fragment was in turn the basis for the poem "The Adventures of TomBombadil", published in 1933, which also introduced Goldberry, the barrow wights, and Old Man Willow (the poem was the source of the events in Chapters 6 through 8 of Book I).
Tolkien introduced Tom into The Lord of the Rings at a very early stage, when he still thought of it as a sequel to the The Hobbit, as opposed to the The Silmarillion (the tone was changed during the first chapters of The Lord of the Rings).
Tom fit the original (slightly childish) tone of the early chapters (which resembled that of the The Hobbit), but as the story progressed it became higher in tone and darker in nature.