TeXbyTopic – Freed ver­sion of the book TeX by Topic

An in­valu­able book, orig­i­nally pub­lished by Ad­di­son-Wes­ley (who have re­leased their copy­right — their ver­sion of the book went out of print in the 1990s).
The book de­scribes it­self as “a TeXni­cian’s ref­er­ence”, and cov­ers the way TeX (the en­gine) works in as much de­tail as most or­di­nary TeX pro­gram­mers will ever need to know.
A printed copy of the book may be had (for a mod­est price) via lulu.com (see the pack­age home page for de­tails).