Oronzo Cilli

An incredible work with much attention to detail and about as exhaustive as could possibly be without being Tolkien himself.

It's a reference book, for sure, so I haven't read it cover to cover, but I read all the prefatory materials, and have made extensive use of it for a paper I have been working on. (The Kindle version makes searching much easier!) Highly recommend for any Tolkien scholars interested in knowing what the professor (most likely) read or at least referred to.