"The Well of the Unicorn" should be recognized as one of the outstanding fantasy novels of the first half of the twentieth century, or any other time. It was first published by William Sloane Associates in 1948, as by the previously unknown author, George U. Fletcher, with a dust-jacket and frontispiece and chapter heading maps by the well-known illustrator Rafael Palacios. That edition was reprinted in hardcover (minus the dust jacket, which itself included an illustrated map) in the Garland Library of Science Fiction in 1975, with the same attribution.
It was in fact the work of Fletcher Pratt (as acknowledged, not very prominently, in the Garland edition). In the late 1940s Pratt (1897-1956; not to be confused with the group "The Fletcher Pratt"!) was a well-known military historian; several of his works in this field are, or were recently, still in print (including one of the others with Palacios maps).
He was also a science fiction and fantasy novelist, alone and in collaboration with L. Sprague de Camp, with whom he also did a series of humorous short stories.
When the Garland edition appeared, "The Well of the Unicorn" already had been published in mass-market paperback under Pratt's name in 1968, by Lancer Books. It was issued again in this format by Del Rey Books in 1976, and, with a new cover, in 1979 -- this last being version currently shown on Amazon.
It was most recently published in a British edition in 2001, as part of the Gollancz Fantasy Masterworks series of trade paperbacks, with the original pagination. This can be found on the Amazon.co.uk site, and, although it does not seem to be readily available in the U.S., is sometimes offered by Amazon directly. (Check for availability on this page!) I have not seen a copy of the edition described here, and can't be sure if "Diane" is an American outlet, the current corporate identity of Gollancz, or a mistake.
I have described the reason (if you can call it that) for the confusing and unnecessary pseudonym, and other details of the publishing history, in a review posted with the Garland edition, which is listed on Amazon, at least for the moment, as by George U. Fletcher. If "Well" is effectively out of print in the United States, in any edition, as you read this, I suggest checking there for some helpful hints on their differences if you are trying to decide which version of it to order used.
If it has gone out of print, with any luck a U.S. publisher will also re-issue the beautifully-produced Sloane edition, as a trade paperback, under Pratt's name, (perhaps without the original publisher's unnecessary prefatory note), in the near future. It should be in print, and Palacios' maps deserve the full size (and often higher-quality paper) of such an edition.
Under any author's name, this is an extraordinary example of fantasy, pre- or post-Tolkien. It is adult in tone, very literate, and intentionally thought-provoking. There is plenty of action, and we meet a variety of appealing (and sometimes annoying) characters. Magic plays a part, but has limits, and its solutions don't always match up well with the problems. Pratt has interesting things to say about a number of topics, from sex to politics, not all of them immediately evident, but clear enough when you start looking for them, in the words and actions of minor as well as major characters. Readers of his "Ordeal by Fire" (otherwise "A Short History of the Civil War," and various combined forms of the title) will recognize some elements. So will those familiar with his less readily available book on medieval Denmark, "The Third King."
Pratt drew on a vast knowledge of military history and political organizations to get the details right, without going into elaborate explanations. His setting has echoes of medieval Scandinavia (his Dalarna is not the Swedish province, but someplace like it), and of the Hanseatic League, but also the Holy Roman Empire. (Neither called by these names, of course; and Pratt's Empire has rather better claims to being Holy, although not exactly Christian, than its historical counterpart.) There are also, I suspect, a few hints of the history of the Balkans. Off in the distance is a quasi-Islamic empire, the realm of "the heathen Dzik," about whom Pratt was planning to write a story, in which, he told his friends, they would have turned out much nicer than reputed by their old enemies.
It sounds like a precursor of more recent fantasy novels with political and military themes and sub-themes. It may be, although you would have to ask their authors. Unlike many of those books, "Well" never preaches at the reader, or tries to offer doctrinaire solutions. (Pratt seems to have felt that inflexible thinking was often a source of problems in itself.) Characters express views, often tied directly to their experiences, and the reader is left to draw conclusions. But it differs markedly from Eddison and Dunsany, whose political theories ranged from pre-modern to non-existence.
"The Well of the Unicorn" is, it should be said, unrelated to Pratt's other major fantasy novel, in a setting resembling Revolutionary France "The Blue Star" (in the omnibus "Witches Three," 1952; separate edition, 1969) -- at least, the geography and system of magic both seem incompatible. The actual sequel to "Well" was one of a number of projects Pratt was planning at the time of his sudden, untimely, death.