Electronic
Use your smart phone as a field guide---it is compact, and packs more knowledge than any book. First, identify some good local resources, and bookmark them to call up while you're in the field. FossilGuy.com is compiled by an active amateur fossil hunter named Jason Kowinsky, who visits and compiles information on fossil sites in the United States and worldwide. FossilGuy.com gives you much the same information (photos included) that you'll find in a more scholarly field guide (see Resources).
A-Z
Bookmark the San Diego Natural History Museum's Fossil Field Guide A-Z at (see Resources). This is a comprehensive, free online resource of photos and zoologic/geographic/paleologic information. It also lists fossils by the type of animal you are looking for--for example, the timber wolf ancestor that you will find in North American sites.
Locality
A guide like the "National Audubon Field Guide to North American Fossils" is scholarly and complete. Still, a localized guide like "Cretaceous and Paleogene Fossils of North Carolina: A Field Guide" (Richard Chandler, 1995) and "Field Guide to New Mexico Fossils" (R.P. Ratkevich, 1978) will provide more specific geological information about a given area.
Smithsonian
Purchase the "Smithsonian Handbooks: Fossils," which covers all continents and fossils. This guide is compact and rich in color photos. Of course, the prehistoric world was vast---Neanderthals populated both what is modern-day France as well as China; sabretooth cats existed on most continents; and snails existed on all continents---therefore this authoritative guide is useful in any location.
Audubon
Purchase the "National Audubon Field Guide to North American Fossils" (presuming you are hunting fossils in Dinosaur National Monument or some other United States or Canadian location). This photographic guide is frequently updated and contains dozens of color photos. It has state-by-state maps, indexes by geographic location and indexes by type of animal.
Geological Surveys
Find a local geological survey online. A local geological survey, commissioned by a country, region or state (like the Maryland Geological Survey of the Calvert Cliffs) will provide you with exactly the finds you should encounter on a site, be it a state park or square mile of desert.