Well, of all the books I've had that have explained IS-LM, i'd go with Frederic Mishkin's "Money, Banking, and Financial Markets". Which... if you can get your hands on it, explains things pretty damn clearly. There's a lot of chapters that explaining stuff that leads up to the IS-LM, and then a few on IS-LM itself. I wish I could do it here, but it's kind of hard to do without diagrams and a lot of time.

Sorry for the late reply.