I would just like to add, that while a songwriting book is helpful, the best way to learn about songwriting, is by actually writing songs.

If you're just starting out, and you don't think you're very good, just write songs for only you to hear. But, keep writing everyday, and explore new ideas with chords and harmony. Eventually, you'll start figuring it out on your own, and your songs will sound better and better.

That's what I did for a long time, before I felt like I could show my music to other people.