"A Guide to Middle Earth: A Complete and Definitive Concordance for The Lord of the Rings" by Robert Foster, 1971
$6.00
Tolkien was completely brilliant at world building. He was so brilliant that, indeed, it can be a bit tricky to keep track of details that are in his books. This concordance helps keep it all straight! This gives very brief entries complete with which books you will find that detail.
Paperback, spine has some minor creasing, but otherwise very good vintage condition.