I've been a songwriter for a long time. Going on 20 years, seriously. Before that I was journaling and well, writing out my pre-teen angst.
Over the years, I've done the good old fashioned trial and error – which is by far THE BEST way to get better writing songs. Yes, I wrote one ok song for every ten bad, then 2 ok songs, then 1 good for ten bad. You get the idea...
But I'd stop for help along the way, pick up some inspiration and generally pick up a few new tricks. So I've narrowed down some of my favourite songwriting books. These are the ones I won't sell, give away or lend without holding someone's driver's license and car keys.
in no particular order my top 3 songwriter books are:
1) Writing Better Lyrics – Pat Pattison
I went to his workshops and also book his workbooks on rhyming etc. This guy is good and has excellent exercises for improving skills. Pat even says John Mayer keeps a copy on his toilet tank. Yah!
2) Writing Music for Hit Songs – Jai Josefs
I did all the exercises at the back of each chapter over the course of a summer. It was alot of work, but well worth it. If filled in a lot of gaps between music lessons over the years and emphasized choosing the write chords and progressions to match the lyrics.
3) How to Make a Good Song a Hit Song – Molly-Ann Leikin
I'm actually re-reading this book and walking around making up rhymes, internal rhymes and generally getting some funny looks from people I walk by on the street.
There are some other books that songwriter friends enjoyed. Frankly I never got as much out of them as the ones above, but you might so here goes:
Anything by Sheila Davis, John Braheny or Jason Blume.
Lastly, you're a bit lost without some help on guitar and found 'Arlen Roth's Complete Acoustic Guitar Guide' was excellent for skill building and Natalie Goldberg's 'Writing Down the Bones', excellent for inspiration and sticking to your calling as a writer.
There was two years's in particular that I read most of the book above and it definitely took me forward leaps and bounds. Of course, you have to put it into practise to get the most out of the learning. Write bad songs! Think of it as clearing them out to make way for the good ones!