The Primer includes units describing the soil food web and its relationship to soil health, and units about bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. From the USDA, Natural Resources Conservation Service.
Includes a simple key to common invertebrates, as well as a photo gallery, and a good bibliography. From 2001, but still useful. From John Meyer's course in entomology. Department of Entomology, NC State University