I'm not sure that such content exists, if it does it would be in a training pack of some sort. Simon Taylor from PTC might know. I was trained by the original developer Advent 3B2 (desktop ALD product) circa 2001.
For simple source edits, you can use the FOM reference to help you understand what methods and properties are available. This reference is part of the APP (ALD) help center that ships with the ALD product and is also hosted online by PTC.
For advanced source edits you would need a deeper understanding of the underlying functionality and capabilities of the APP engine and how Styler is building the APP code. At the extreme end of the scale you can actually roll-your-own APP/ALD template without using Styler at all, but you have to be expert-level for that 🙂
Not sure if that helps but I'd suggest starting out by creating your own simple XML+DTD/Schema and making an Arbortext doctype. Then you could create a brand-new Styler stylesheet to match your sample XML. Finally you would be able to examine the APP code it produces and make changes informed by the FOM reference in order to deepen your understanding of the product, its configuration, and customisation.