Elbroch, Mark. 2003. Mammal Tracks & Sign: A Guide to North American Species. Mechanicsburg, PA: Stackpole Books. This is the best animal tracks and sign field guide that I have found. The only drawback is that it is a rather heavy book for carrying in the field.
Halfpenny, James, C. and Jim Bruchac. 2001. Scats and Tracks of the Northeast (Scats and Tracks Series). Guilford, CT: The Globe Pequot Press. This book is part of a series for different parts of the US. It is a good companion field guide, which can easily be carried into the field.
Rezendes, Paul. 1999. Tracking and the Art of Seeing: How to Read Animal Tracks and Sign. New York, New York: Harper Collins. This is less of a field guide and more of an informational book on different common species, their behavior, and the tracks and sign they leave behind.
Brown, Jr., Tom. 1986. Tom Brown’s Field Guide to Nature Observation and Tracking. New York, New York: Berkley Books.
Brown, Jr., Tom. 1999. Tom Brown’s Science and Art of Tracking. New York, New York: Berkley Books.
Young, Jon and Tiffany Morgan. 2007. Animal Tracking Basics. Mechanicsburg, PA: Stackpole Books. I grew up reading Tom Brown’s books, but Jon Young’s book is better organized and more clearly written.
These books are a bit pricey and only have a few chapters that are pertinent to lost pet investigations, but I hope to purchase them some day. The second book is probably most useful for its chapter on “The Use of Hair Morphology in the Identification of Mammals.”
Cooper, John and Margaret Cooper. 2013. Wildlife Forensic Investigation: Principles and Practice. CRC Press.
Huffman, Jane and John Wallace. 2011. Wildlife Forensics: Methods and Applications. Wiley.
Merck, Melinda. 2012. Veterinary Forensics: Animal Cruelty Investigations. Wiley-Blackwell.
At the very least, you should be able to easily differentiate between different animal skulls.
This is a great little guide. However, be aware that it is a dichotomous key, so you need to learn the different parts of the skull in order to work your way through the manual. Key-Guide to Mammal Skulls and Lower Jaws by Aryan Roest.
If you are more of a visual person, then you might prefer this guide: Animal Skulls: A Guide to North American Species by Mark Elbroch.
Samples of animal fur/hair may be found during a missing pet case. Most often these are associated with the recovery of animal remains or in the feces of a predator. It is possible to identity most animal hairs to the level of genus (i.e. Canis could be dog, wolf or coyote) and in some cases species (i.e. domestic dog from coyote). Identification to the level of individual is more of an art than a science and the unknown sample can generally only be determined to “be consistent” (or not) with the known sample. DNA tests are needed for a higher degree of reliability but these are significantly more expensive. For services, see Forensic Investigations under the Resources section.
Alaska Fur ID Project – includes dog
FBI: Forensic Hair Comparison (focuses primarily on human hair but good background material)
Hair ID (Ecobyte) – Hair identification software for Australian Mammals