Petersen's is the best bet for field guides, although you need two. Petranka is a must-have, but not practical for a field guide, and a bit dated now. Urodeles du Monde is comprehensive, but too big for the "field", and it's in French. Salamanders of the Southeast and Salamanders of Ohio are two excellent backups that cover a lot of eastern species. Otherwise, I try to have multiple field guides for each state, park, and region I'm in. Each guide has different style, coverage, map types, images, and information. The smaller the region covered by the guide, the more information it tends to have which is specific to that region. This is only practical if you have a vehicle with space, and the time or money to build up the library!