Elementary introduction to creating BibTeX styles?

The document by Nicolas Markey Tame the BeaST is a very good reference. It contains a general presentation of BibTeX, and chapter 4 is devoted to editing .bst files. The overall structure of a .bst file is explained, as well as the Reverse Polish Notation used by bibtex, and the main functions are described. As far as I'm concerned, I've always found it very helpful.

Otherwise there is also Designing BibTeX files, by Oren Patashnik, the author of BibTeX (I don't know if there is a "canonical" link to this pdf, if so please feel free to edit my answer).

there should be some documentation in your local TeX installation:

$TEXMFDIST/doc/bibtex/base/btxdoc.pdf
$TEXMFDIST/doc/bibtex/base/btxhak.pdf
$TEXMFDIST/doc/bibtex/tamethebeast/ttb_en.pdf

and if you are familiar with german:

https://archive.org/details/pdfy-DTVSHmzsr3tqPmMh, which is an archived version of the document formerly at http://archiv.dante.de/dante/events/dante2002/handouts/raichle-bibtexprog.pdf