BibTeX.org - Quick introduction to BibTeX and in-depth information. - http://www.bibtex.org
The BibTeX Format - Standard LaTex bibliography reference and publication management data format. - http://www.ecst.csuchico.edu/~jacobsd/bib/formats/bibtex.html
The NatBib manual - The Natbib package that comes bundled with LaTeX provides better citation features. Primarily used for authors wishing to use the (Surname, year) format, such as the Harvard style. This manual is written by the Natbib author and is a very clear guide on h - http://www.cmis.csiro.au/Graham.Williams/TeX/docs/natbib.pdf
Help on BibTeX - Online help for BibTex from Norman Walsh. - http://nwalsh.com/tex/texhelp/BibTeX.html
BibTeXing - A BibTeX manual written by Oren Patashnik (co-designer of BibTeX). This is a fairly thorough guide, as you would expect, and provides wide coverage of the BibTeX file format. However, this is more reference than tutorial. - http://newton.ex.ac.uk/tex/pack/bibtex/btxdoc/btxdoc.html