Sources for tree bark identification include TreeBarkID.com. This site hosts a comprehensive list of trees that can be searched for bark identification photos. The site focuses on the best characteristics to consider when attempting to identify trees in the lower Midwestern latitudes.
Each listing provides the name and description of the tree, with a photo gallery of bark pictures and suggestions of trees that may be related. The site provides a map of the areas covered and a bark identification key that features clues as to what to look for in branching, how to rule out certain species, and pertinent questions to ask when attempting identification.