Back in 2013, Simon Taylor put together a book for a course on Arbortext Advanced Print Publisher (now known as Arbortext Layout Designer) for the Arbortext Training partners. It included everything you needed to know about programming for APP.
It includes instructions on APP tags, page components, content types, style components, variables/counters/and FISH, conditional text formatting, how text processing is done, using JavaScript in APP and in Styler.
A lot of the things that are not documented well in the Help Center are included in here.
The course materials were created against APP 11.0 M030 and is compatible with current releases (which have only expanded from here).
Download the book
An Introduction to APP for Styler Users (PDF)
View the slides