<1> An Introduction to the Tracks and Signs of Familiar North American Species James Kavanagh.
The Pocket Naturalist series is a great resource for lightweight, sturdy field guides. Although they rely heavily (okay, entirely) on sight identification, and some of the colors are a bit over-exagerated, they are great to have along for a quick identification. Being laminated, they hold up particularly well along the coast or in a boat, kayak, canoe and in muddy areas.
Laminated fold-out guide, 12 panels: 4 x 8 1/4 folded, 22 x 8 1/4 unfolded, full-color illustrations, June 2000