Tolkien Books feature

The Works:

• The Hobbit
• The Lord of the Rings - I. The Fellowship of the Ring, II. The Two Towers, III. The Return of the King (note: there are MANY editions)
• The Silmarillion (Middle-Earth Pre-History)
• Unfinished Tales of Numenor and Middle Earth (Previously unpublished details of the History of Men and other stories of Middle Earth)

The "Making of" Books (edited by Christopher Tolkien):

• The History of Middle-Earth: Part VI - The Return of The Shadow (The History of The Lord of the Rings, Part I)
• The History of Middle-Earth: Part VII - The Treason of Isengard (The History of The Lord of the Rings, Part II)
• The History of Middle-Earth: Part VIII - The War of the Ring (The History of The Lord of the Rings,Part III)
• The History of Middle-Earth: Part IX - The End of the Third Age (The History of The Lord of the Rings, Part IV)

Biographies:

• J.R.R. Tolkien, A Biography by Humphrey Carpenter
• J.R.R. Tolkien: Author of the Century by Tom Shippey
• The Letters of J.R.R. Tolkien by J.R.R. Tolkien

Literary Criticism:

• The People’s Guide To J.R.R. Tolkien - TheOneRing.net Staff
• More People’s Guide To J.R.R. Tolkien - TheOneRing.net Staff
• The Road to Middle-Earth by Tom Shippey

Other:

• The Maps of Tolkien's Middle-Earth by Brian Sibley and John Howe
• The Tolkien Companion by J.E.A. Tyler (a Tolkien Encyclopedia)
• The Atlas of Middle-Earth by Karen Wynn Fonstad (best cartographic collection of Tolkien’s word - bar none)
• The Origins of Tolkien's Middle-Earth For Dummies by Greg Harvey (good straight-forward simple overview of Tolkien's work)