Finally!
Finally there is a post where I can really feel like an expert!
I've been reading on songwriting for about seven years and, for strictly lyric work, I would suggest "The Craft of Lyric Writing" by Sheila Davis.
She sticks to the topic at hand and veers towards writing mostly literary (read:intelligent) lyrics. Some of the great examples she uses that struck a chord with me were; Dan Fogelberg - Same Old Lang Syne, a rupert holmes song that currently escapes me and, by my favorite songwriter, Harry Chapin's Sniper. She gives a lot of do's and don'ts that you might not have considered hard and fast rules but make sense when she discusses them. (example; how to evoke an empathetic protagonist)
This book was especially helpful for me because it dealt mostly with the music I want to do (ie, story songs).
I've also read Jimmy Webb's "Tunesmith" if you're considering that. It's got some helpful hints for lyrics but his music talk gets awfully technical. The best advice, and more or less universal among songwriting books, is the first songwriting tool you buy is a rhyming dictionary. I use an online one when writing at my computer but have a hard copy that I find invaluable.
And, on an end note, I would suggest "And then I wrote" as a good bathroom book for songwriters. Little tidbits on the craft while you take care of other things...keeps you focused even during your breaks.
I've also read dozen others but those are the three I would recommend starting out. They're the one's that have helped me the most in my writing.
Good luck sjag,
miles maxwell