Narrative Books:

• The Hobbit
• The Lord of the Rings
• The Children of Húrin
• The Silmarillion

Slightly academic:

• Unfinished Tales
• Beren and Lúthien
• The Fall of Gondolin
• The Fall of Númenor

Heavy academic:

• The History of Middle-earth
• The Nature of Middle-earth
• The Letters of JRR Tolkien
• The History of The Hobbit

Biographical Books:

• A Tolkien Biography - Humphrey Carpenter
• J.R.R. Tolkien: Author of the Century - Tom Shippey
• Tolkien and the Great War - John Garth