Reddit mentions of Tonal Counterpoint for the 21st-Century Musician: An Introduction

I've played a decent amound of rock, (mostly into Wilco, Sufjan type stuff) and I feel that playing Bach helped me a lot for writing memorable parts with good voice leading. It's mostly about giving yourself a solid harmonic framework to go off of. Like building a house... you can kinda do whatever you want with the decorations, but the framework and structure has to be there. Bach gives us that framework, even for rock/pop/jazz (to an extent).

Some things of his that might help you for guitar parts: his solo violin (and solo cello) stuff. He was able to coax polyphonic sounds and a sense of harmony out of two instruments which are mostly monophonic, and you really learn how to write a good melody. For two-part structure (bass+melody, the most important voices), check out his Inventions, and for 3-part, check out the French Suites. For heavier stuff, check out the Well-Tempered Clavier or B Minor Mass. It's mostly about being aware of how you're moving the voices, and how your parts are moving melodically... thinking of harmony as melody.

Obviously, there's a huge difference between Bach and something like funk or afrobeat, which are groove-based, but I think studying him is really helpful for writing parts that "just fit" with the rest of the band, or knowing how to keep strong harmonic structure with minimal instrumentation (solo, duo, etc).

Some other books to maybe consider:

Exploring Jazz Arranging (He also talks about Bach)

Contemporary Counterpoint

Tonal Counterpoint for the 21st Century Musician

Voice Leading: The Science Behind a Musical Art