A Document Preparation System: LaTeX,
by Leslie Lamport, ISBN 0-201-52983-1, published
jointly by the American Mathematical Society and Addison-Wesley
Publishing Company. The 2nd edition, 1994, describes LaTeX2e, the
second widely distributed version of LaTeX. The first edition of this
book, which appeared in 1985, described LaTeX 2.09.
The standard reference for what goes on inside TeX is
The TeXbook
by Donald E. Knuth, ISBN 0-201-13448-9, published jointly by
the American Mathematical Society and Addison-Wesley.
M. Goossens, F. Mittelbach, and A. Samarin,
The LaTeX Companion, published
by Addison-Wesley, ISBN 0-201-54199-8 (essential for the serious
LaTeX hackers).
L. Botway and C. Biemesderfer,
LaTeX Command Summary, published by
the TeX Users Group, Providence, RI is a good companion.
A. Diller,
LaTeX Line by Line, published by Wiley.
N. Walsh,
Making TeX Work,
published by O'Reilly & Associates, ISBN 1-56592-051-1.