The OP also asked about a guide for writing slackbuilds.
For me the most useful find in this area was the
slackbuilds.org templates page. To the newcomer there are so many esoteric little things to worry about when compiling software in general, but also about "best practice" when it comes to building for a slackware system in particular. These templates not only point out all the things you need to be mindful of, but also how best to deal with them for different compilation paradigms (automake, cmake, python, etc).
Cheers,
Michael