Any of Bill Hylton's books. My personal favorite is "Router Magic", just because of the router lathe. It is the most advanced of all his books. It's been out of print for a while, but it shows up on eBay and Amazon fairly regularly. It will cost around $30, for a copy in good shape.
"Woodworking With the Router", is probably the best book out there for the new to intermediate user, and I highly recommend buying it before you buy your first router, because he devotes space to explaining what features you want, and which features you want to avoid, that even a neophyte can get a quality tool, the first time out. He also explains what features a router table needs and doesn't, (a miter slot).