Which would you choose? Keep in mind this would be about Tolkien or his works, as opposed to Tolkien's writings themselves.
There are a lot of choices available. Some good contenders include:
* The Atlas of Middle-earth by Karen Wynn Fonstad
* The Complete Guide to Middle-earth by Robert Foster
* The Complete Tolkien Companion by J. E. A. Tyler
* J. R. R. Tolkien: A Biography by Humphrey Carpenter
Which one stands out for you, and why?