Guide to Literary Agents: Where to Find the Right Agents to Represent Your Work
This volume from Writer’s Digest Books does exactly what it says. It helps writers locate agents. You may find a copy at your local library if you’re not ready to buy it yet.
But the best part of this site is the blog. Here you will find a series of interviews with agents from various agencies providing background on the agent and the kind of works they are looking for. Click the Editor's Blog icon in the upper left.