As spring is upon us here in the Midwest and the ferns are starting to emerge, I thought I'd mention the Peterson Field Guide to Ferns by Boughton Cobb, Elizabeth Farnsworth, and Cheryl Lowe, which is my go-to guide for ferns in the field. It covers Northeastern and Central North America and is a pretty exhaustive catalog of the species to be found in the region. The book has keys for ferns in general and then for each genus, and a black-and-white drawing of each species. There are also color photographs of the most common members of each genus, although I wish there were more of these (color photos, not common species, although that would also be nice...); often I find these more helpful than line drawings for identifying things.
This is a great book to use in the field, but it's important to note that it often asks you to look at characters such as rhizome habit or the lowermost pinna pair along a frond, so it isn't the best if you're trying to identify a pressed specimen or an individual frond you find lying along a roadside. But if you're out in the woods and come across a living plant you can't identify, this is a great book!