The Peterson Field Guide to Ferns is excellent. It covers ferns as well as clubmosses, horsetails, spikemosses and quillworts, and also has essays on life cycle and other topics, even gardening with ferns. But I was sometimes frustrated in using the keys and illustrations to make positive identification.
I found a neat little guide that solves that problem: Fern Finder, by Anne C. and Barbara G. Hallowell (Nature Study Guild Publishers, PO Box 10489, Rochester, N.Y. 14610, www. naturestudy. com). I got mine from Amazon.
It's absolutely idiot-proof, and its small size makes it easier to take into the field.
McGraw-Hill publishes a series called "How to Know The ...." which are incredibly detailed keys for identification. I have the volumes on Lichens and also Mosses and Liverworts, but, as an amateur, find them much too difficult to use.