Indeed, you just need a little music theory. Some recommendations:
Music Theory and Arranging Techniques for the Folk Harp by Sylvia Woods
Cool Chords and Groovy Rhythms: Arranging for Lever Harp in Modern Styles by Verlene Shermer. I think this one is exactly what you are looking for.
3’s A Chord by Ray Pool
Clever Levers: Harmonic Exercises for Advanced Lever Harpists, also by Ray Pool. Not an easy one to grasp but it will make you very familiar with Chord progressions, Diatonic Seventh Chords, Harmonizing scales with a triad, and the basics of Lead Sheet playing (which is format of music in fake books).
There are also books specifically on playing from fake books such as: How To Play From a Fake Book by Blake Neely. Although for keyboard a lot of the basic concepts are the same.
Also any introductory book on Jazz Theory will deal with Chord Substitutions. Switching out chords is a fundamental aspect of Jazz.