BibTeX, a program originally designed to produce bibliographies in conjunction with LaTeX, is explained in Section 4.3 and Appendix B of Leslie Lamport’s LaTeX manual. The document “BibTeXing”, in the BibTeX distribution (look for btxdoc), expands on the chapter in Lamport’s book. The LaTeX Companion also has information on BibTeX and writing BibTeX style files. (See LaTeX books for details of both books.)
The web site “Your BibTeX resource” offers a solid introduction, but doesn’t go into very great detail.
The document “Designing BibTeX Styles”, also in the BibTeX distribution (look for btxhak), explains the postfix stack-based language used to write BibTeX styles (.bst files). The filebtxbst.doc, also in the BibTeX distribution, is the template for the four standard styles (plain, abbrv, alpha, and unsrt); it also contains their documentation.
A useful tutorial of the whole process of using BibTeX is Nicolas Markey’s “Tame the BeaST (The B to X of BibTeX)”, which may also be found on CTAN. A summary and FAQ by Michael Shell and David Hoadley, is also to be recommended.
This answer last edited: 2013-10-15