I've loved Fitzgerald's Omar ever since I bought a remaindered World Books' edition for one shilling in 1969. That had an introduction by Laurence Housman, younger brother of the poet, which, for me, has always been a model of expository clarity, with one exception: no mention was made of Fitzgerald's seemingly repressed homosexuality which, in some ways, was the begetter of his translation. Daniel Karlin's sensitive, beautifully written introduction remedies this, and provides the best short summary of the poem's genesis, writing, and meaning that I have read (and I've read quite a few). The text of the first edition is used (Fitzgerald produced five versions over 30 years, the last - published immeduately after his death - containing very minor revisions) and is very well presented; but the great strength of this book lies in the other material. As well as the previously mentioned introduction, there's a chronology, a publication history, a bibliography, Fitzgerald's own notes, notes on variant readings, some early critical responses to the poem - including Tennyson's poem "To E Fitzgerald" - and Karlin's very full notes on the text. And if all this sounds dry - it isn't. I found it quite enthralling. If you already know the Rubaiyat then you need this volume; if you don't yet know it, treat yourself. You won't regret it.
One thing missed - Karlin notes how Fitzgerald would "alter" poems, plays and prose written by others; he believed he was making them more readable, cutting out the padding. He physically altered books in his own library, cutting out chunks he didn't like and having the books re-bound. Not so well known is that he did the same with paintings - he would overpaint parts of pictures in his collection so that they accorded more with his taste. Odd but true.