``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]
TeX by Topic, A TeXnician's Reference,
by Victor Eijkhout
Addison-Wesley Publishing Company
Wokingham, England 1992
List price US$28.95
This book went out of print somewhere in the 1990s, after which the publisher returned the rights to me. In 2001 I made it available for free download. Right now it’s available under the Gnu Free Documentation License. You can also get a nicely printed and bound copy from lulu.com for $14.04 plus shipping. There is a Nook version available from Barnes and Noble.