Well, for starters, if you can't play guitar or piano, you don't have much of a choice except to write on bass, now do you?
Songwriting, for me, has always been easier on chordal instruments. It's easy to write basslines on bass, but hard to write entire songs. That's been my experience.
Finally, as long as you don't take exception to the name of the book, there's always "Songwriting for Dummies".
