You will find a nice free documentation on LaTeX with the title The Not So Short Introduction to
LaTeX2e on CTAN in the directory :
ftp://ctan.tug.org/tex-archive/info/lshort.
This introduction is available as dvi, PostScript and pdf file. There is also a German version in
the directory :
ftp://ctan.tug.org/tex-archive/info/LaTeX2e-Kurzbeschreibung.
Of course, there are various books available on LaTeX. For beginners, you might take a closer
look to
L. LAMPORT LaTeX: A Documentation Preparation System User's Guide and Reference
Manual
H. KOPKA and P.W. DALY A guide to LaTeX2e
For more advanced users, the following books will be of valuable help:
M. GOOSSENS, F. MITTELBACH and A. SAMARIN The LaTeX companion
M. GOOSSENS, S. RAHTZ and F. MITTELBACH The LaTeX graphics companion