Buying songwriting books and learning materials can be confusing. There's a lot out there, and sometimes the books don't directly apply to your needs.
You might not care about "How to Write a Hit Song," you just want to figure out some great chord substitutions to make your songs more interesting.
You don't want to learn "Writing Songs for Keyboards," because you play the guitar. Or, you play Keyboards and know how to write for keyboards already and want to write for guitar.
You don't want "Theory for Dummies," because you already know the basics of theory.
...or something like that.
Well, hey, I'm not sure if you'll find that perfect book. Personal lessons can really fill in that gaps. Also, you might need to buy a bunch of books and just dive into the material.
I recommend learning how to write one's songs down. It's basically the litmus test for competent musicianship as a songwriter.
Read more about learning how to write your songs down.