When I got my serger I checked some books out of the library, as I don't have much experience with them. The library doesn't have a very comprehensive collection of sewing books (more's the pity), but there were a couple that were promising.
I was a big fan of The Complete Serger Handbook by Chris James. The instructions were clear and useful, and she is a woman after my own heart, the slapdash sergist if you will. She illustrated concepts with several different models of serger, and gave detailed instructions on stitch width, length, and tension for various techniques. I am almost tempted to actually *gasp* buy this book. The amazon listing is for a paperback, which wouldn't be as useful; the one in the library was hardback spiral bound.
I didn't find Creative Serging by Nancy Bednar and Anne van der Kley as useful, mostly because they are writing for top-end machine owners and were most delighted in the coverstitch function. My machine doesn't have coverstitch so at least 80% of the projects and instructions were inapplicable to me. However, their section on flatlocking added a bit to what Chris James had said and it made a little more sense to me.
I also checked out Second-Time Cool: The Art of Chopping Up a Sweater by Anna-Stina Linden Ivarsson, Katarina Brieditis, and Katarina Evans. Some of the projects are a little too cool for me (it was in the Young Adult section, and the YA librarian looked at me a little suspiciously when I entered his section), but it gives good information in an easily understandable way. There were a few really clever things, like their method for making yarn buttons, and lots of inspiring pictures. They also tied their work to what's on the runway, which I appreciated. I went to the thrift store on Friday before I went to Ikea and got a bunch of wool sweaters and am enjoying playing around with them.