The Lord of the Rings: A Reader's Companion

It's a treasure trove for hardcore fans for sure. I'd even say it is mandatory if you're the type of fan that has read The Lord of the Rings multiple times, and plan to read it again. It's 894 pages, with 15 pages of bibliography and a 64-page index.

It has a lengthy introduction on the history of the book's origin and publication history, notes on the chronologies, calendars and moons, and some pages on the different maps. It also includes 30 pages of notes on the nomenclature, written by J.R.R. Tolkien himself. The main part follows the text, and offers thousands of notes - some trivial and short, others long, quoting from letters or earlier drafts, and from other scholars, including Christopher Tolkien's extensive work.

It is meticulous about different ways of spelling dwarfs/dwarves/Dwarfs/Dwarves and printing errors in various editions of the book, should stuff like that float your boat. There's also an index of all the changes that were made for the 2014 60th Anniversary edition by HarperCollins, but that would be interesting for scholars only.

Anyhow: you do not need it as a casual reader, not at all, but it is a tremendous achievement in itself.

If you're interested in that kind of thing: read my 7000+ word analysis on free will & determinism in LOTR on Weighing A Pig via the link below: