Title: TEX by topic
Description: ``TeX by Topic is a systematic reference manual designed as a companion to tutorial guides on the system. Packed with original, practical ideas, this book provides an invaluable information source that will give the TeX user the essential insight needed to solve TeX problems.'' [From the back cover] 

A beautifully written, concise and comprehensive discussion of TeX as a programming language [...] Eijkhout's book is not a loose collection of unconnected entries but a very systematic account of how TeX works as a programming language. Not the ideal first introduction, but an absolute must-read if you want to understand the deeper foundations of the TeX language. On many occasion when one of my macros didn't work, only by consulting Eijkhout's book was I able to find the source of my problem. [From a review by Martin Diedrich] 

This book is out of print, but now available electronically and from lulu.com: http://eijkhout.net/texbytopic/texbytopic.html
Subjects: Computer programs, Computerized typesetting, Mathematics printing, TeX (Computer file)