All the music in the Rainbow Guitar books works just as well with a plectrum as with fingerstyle. Click here for a short guide, which explains how to approach the books as a plectrum player.
At the very simple level of Book 1, the plectrum player can approach the pieces in just the same way as the pupil plucking with the thumb. The only difference is that the thumb normally plucks just downwards, whereas the plectrum can play with all downstrokes or with a down-up movement.
In Book 2, many of the pieces feature larger string jumps. They are all playable with a plectrum, but the plectrum player will need to master a variety of strokes and approaches.