There's a shortage of good books on songwriting. That's one of the reasons I started this blog. Most songwriting books dance around the subject; they never quite get around to how to write a song. Some other songwriting books focus narrowly on writing hits for pop radio. There's nothing wrong with pop radio hits, but it's a tiny, tiny part of a big and beautiful world of music -- like looking out through a keyhole instead of opening the door and stepping outside. If you're a rock band, a singer-songwriter, or a musical artist of any kind, you have to take these hit-oriented books for what they're worth, and disregard advice that doesn't apply to your music.
I previously recommended The Artist's Way by Julia Cameron. It's a rigorous DIY creativity course. If your creativity isn't flowing freely, give yourself the time for The Artist's Way.
If you want to write better lyrics, my top recommended book is not about lyrics at all; it's about poetry. Get yourself a copy of The Art and Craft of Poetry by Michael Bugeja. If you had to suffer through stuffy, old, pedantic poetry books in school, don't worry -- this is nothing like those. It's a modern, practical guide to help you write better poetry, and you can easily apply its lessons to song lyrics. Used copies are 99 cents at Amazon -- can't beat that price.
Hugh MacLeod has written an intriguing book called Ignore Everybody: and 39 Other Keys to Creativity. I haven't actually read the book yet, so I can't tell you whether it lives up to its promise. But I definitely recommend reading this page, which gives away some of the best ideas from the book. Sample advice: "Don’t try to stand out from the crowd; avoid crowds altogether."
I like Hugh MacLeod's ideas; they definitely fit with my own exhortation, "Create something unexpected." You're not here to compete with other songwriters; instead, you have your own creative mission to discover and fulfill on.
Robin Frederick has a long resume of writing songs for other artists and producers, and just as much experience critiquing songs written by other songwriters. So when she offers songwriting advice, it's practical and it gets to the point.
I recommend reading her page on How to Write a Song (aimed at beginners but worth a read for anyone), and following her on Twitter for concise, helpful songwriting tips.
Robin Frederick's book Shortcuts to Hit Songwriting: 126 Proven Techniques for Writing Songs That Sell sounds promising. Despite the emphasis on hits, I expect that it's loaded with immediately useful ideas, techniques, and tips.