I usually don't post in this forum much, but I've seen alot of "songwriting book" Qs posted here lately.
IMHO: no book can ever teach you how to write a song. A book can only give you ideas for structure, lyrical scheme et all.
Your best resources, IMO, are rhyme and thesaurus resource books. Rhyme: in case you get stuck somewhere (I personally avoid rhyme alot) and thesaurus to find a different or better word choice - it's really cool as a different word choice can change the entire direction and/or feel of a song. Random word generators are also a cool technique.
As for what comes first, lyrics or music: some people have hard and fast rules for this either way - others just take it as it comes. Lately, I'm working out the music first. It's a rare and inspired occasion when it all happens together.
Good luck.