A list of the best books on matters Tolkien. For a bibliography of the Medieval period in general we recommend the list at Eugene Fairfield's site.
by The Master
by His Students
by His Predecessors
This is the work that was most important to Tolkien, the life's work he began long before The Hobbit and continued until his death. Not a novel, this is the mythology of England (which is Middle Earth) that Tolkien had aspired to create. An absolute must for every fan or student.
In case you don't have your own copy, here's the link. Don't worry about getting it in one volume. Tolkien actually wanted it that way, it was only split into the traditional three books because the publisher demanded it.
Contains the complete text of the Out-of-Print volumes Adventures of Tom Bombadil and Tree and Leaf, an essay and a story on what he called "fairy stories" that is crucial to understanding what Tolkien was trying to do. Also includes Farmer Giles of Ham and The Homecoming of Beorhtnoth Beorhthelm's son
Includes stories of the Istari (the "wizards" of the Lord of the Rings), the hunt of the Nazgul for the ring, accounts of the Numenoreans, variant versions, and many other items of historical interest for the student of Middle Earth.
The first volume of HOME, The History of Middle Earth, which, taken together, contain nearly everything Tolkien wrote about it. In later volumes there are amusing revelations (such as that "Aragorn" was originally a horse's name, and Aragorn's name was "Trotter." This volume covers much of the origins of the world.
J.R.R. Tolkien: Author of the Century by Tom Shippey
An exhaustive study of Tolkien, by the professor who sits in his chair at Oxford. Shippey covers everything, from the Lord of the Rings to the Silmarillion, The Adventures of Tom Bombadil, and Leaf by Niggle, he discusses it all.
The Road to Middle-Earth by Tom Shippey
A more scholarly version of Shippey's great study. If you are determined and serious about your Tolkien research, this is the book you must read. The other book is the easier-reading version commissioned on account of the movies.
J.R.R. Tolkien: A Biography by Humphrey Carpenter
The definitive overview of the life of the Grand Master of Fantasy.
The Letters of J.R.R. Tolkien by Humphrey Carpenter
A collection of his letters pertaining to his works, including his answers to fans questions. Contains many insights into his works documented nowhere else. An essential.
Beowulf: A New Verse Translation by Seamus Heaney
This is it, from the horse's mouth. A real medieval work, talking about medieval doings. That it is a tale of monsters and mythical heroes is of no concern, the passing of the cup, the challenge at the gates, the boasting, the giving of arms, it's all here, Anglo-Saxon in all it's glory. (Gory?)
The Glittering Plain by William Morris
A fascimile of the original Kelmscott Press edition, an extraordinarily beautiful book. Allegedly, Tolkien was inspired by this book to create one of his most beloved characters: Gollum.
Norse tales of dragon-slaying and heroism. Stephan Grundy retold this for his recent Rhinegold, C. S. Lewis may have been inspired by this dragon for the one in Voyage of the Dawn Treader, and this is the very sort of tale Tolkien complained of not having in English, the complaints that lead him to write.
One of the great Icelandic sagas. Another of the ancient texts that made Tolkien jealous of of the Norse, and inspired him to his Master Work.
If you have suggestions of more great books, tell us about it at the Numenor Council.