Jeppson, L.R., Kiefer, H.H. & Baker, E.W., 1975
Mites injurious to economic plants
Keys to groups and genera.
Contains many microscopic drawings of gall mites etc which can help distinguish different species on the same host, at least if the host happens to be of economic importance.
The scope is world-wide and many of our crop and ornamental plants are included.
Available for reading online, although not for downloading.