While I am a regular user of Twitter, I've always felt that somehow I wasn't utilizing the site to its fullest potential. Twitter is fun and all, but there's so much more to it than the usual "Going out to lunch now."
Geekpreneur's "99 Ways to Make Money Using Twitter" is a fun book to read and get ideas from, and it's more than just the typical "ideas book" flooding the market everywhere. Besides explaining what the idea is, each method the book gives has a section on how to do it, what to do to get started, and some tips to get you closer to success. At the end of each entry is a model to follow, which links to a Twitter account so you can see exactly what an actual person is doing. You don't see that in every idea book.
I'm already starting to implement the strategies in this book, and I think it'll really make a difference in my wallet as well as my Twitter account itself.