Best book (s) on advanced sail trim

[Flint]

Look for these on Amazon in print or Kindle:
Felix Marks

https://www.amazon.com/Jib-Trimming-.../dp/B004FV54YC
https://www.amazon.com.au/Mainsail-T.../dp/B004LB4RFS

IMHO: Whether you're sailing a racer or a tub, the more you know about how to handle your own rig, the better you'll sail, and the more you'll enjoy.

best of luck

[AKA-None]

The other thing is to just try stuff. I practice this a lot to see what happens. And sometimes the boat goes slower and sometimes it heels more and then I know that if that isn't what I wanted then remember what I undid before.

[td0tz]

Speed Sailing, Gary Jobson and Mike Toppa