Here is some of what I’ve picked up around the forum.
@JoshFossgreen recommends this book for music theory.
All About Music Theory By Mark Harrison
Josh also recommends this website.
musictheory.net
Many people on BassBuzz have bought this book.
Music Theory for the Bass Player (arisbassblog.com)
@gcancella started this thread and created the below web site for exploring scales, modes, chords, and intervals.
https://fretful.io/ .
@juli0r started this thread and created the below website for practicing your scales.
Groove and Scale Workout (for Beginner and BadAsses)