A while ago, I listed my favorite books about comedy writing, but usually it's a good idea to read as many books on a subject as you can. As such, here's a couple more I'd like to recommend. They aren't my personal favorites, but they're useful enough that they're worth a read.
The Comedy Bible, by Judy Carter
This one's also geared around standup, but focuses on a particular style that may not be for you if you like flexibility in your humor. Still, the book offers great information about writing and improving your act, with lots of helpful tips from established comics sprinkled throughout the text. There's even a large section on finding different types of work and marketing yourself. If you're looking for a thick, comprehensive all-in-one manual, this is the book for you.
Damn! That's Funny! Writing Humor You Can Sell, by Gene Perret
You can tell I like this guy's books, can't you? This one deals with writing funny articles and filler for magazines and the like. Not only does it explain how to write humor articles such as essays and parodies, but the first part of the book covers a lot of places you can mine for topic ideas, which makes this book valuable to me. If you find it cheap, go ahead and pick it up.
The Comic Toolbox, by John Vorhaus
This book is exactly what it says on the tin - though the main thrust is about comedic storytelling, its main feature is to offer a variety of techniques to help enhance comedy. At the time I got this, I didn't find it all that useful - I still believe you need to understand the basic mechanics of writing humor, and Gene Perret's books do a much better job of explaining this. However, for humorists who like tinkering with "optional rules" (for you Dungeons and Dragons fans out there), it covers a lot of classic comedy storytelling techniques you can plug in when neccessary. If you're less of a joke man and more of a classic author, this book is worth a look.
Of course, I always advocate giving books a read before committing to a purchase, especially reference books like these. You don't want to blow a lot of dough on something you may not need in the future, do you? (You'd be surprised how often this comes up...especially when it comes to yours truly!)
Still, see what sticks out for you. Everyone's different and works best with different things. A classic rule of comedy and life. Keep at it!