Goossens, Michael & Mittlebach, Frank, & Samarin, Alexander;
Addison-Wesley; 1994; ISBN 0-201-54199-8; 530pp.
`If you are one of those users who would like to know how LaTeX can be extended to create the nicest documents possible without becoming a (La)TeX guru, then this book is for you' --- from the Preface. Bruce Thompson adds: "A very nice book providing a lot of information about the new extensions to LaTeX, provides a large number of examples showing precisely how your document's layout can be manipulated"
Lamport, Leslie; Addison-Wesley; 1994; ISBN 0-201-52983-1; 256pp.
Bruce Thompson: "The ultimate reference on LaTeX 2.09 by its author. A new edition covering LaTeX2e (the version included in the current TeX/LaTeX distribution) is in preparation. LaTeX 2.09 is fully supported by LaTeX2e. A must for anyone wanting to use LaTeX. Provides a gentle introduction to document preparation and the various tools that LaTeX provides for producing professional quality documents. Lots of examples."
Addison-Wesley; 1986, ISBN 0-201-13448; 496pp. See http://www.awl.com/cp/TeXbook.html
Bruce Thompson: "The definitive user's guide and complete reference manual for TeX. Probably not needed for casual LaTeX use, but a fascinating book nonetheless." I'll strengthen that by adding that this book is not for the faint of heart.
Knuth, Donald A.; Addison-Wesley; 1986; 0-201-13444-6, 1986; 384pp.
Bruce Thompson: "The definitive user's guide and reference manual for METAFONT, the companion program to TeX for designing fonts. An excellent work if you're planning to design your own fonts for use in TeX and LaTeX. METAFONT is included with the normal TeX/LaTeX distribution." This book is definitely not for the faint of heart.