Freely available at this Google Books link.

Although quite old, and likely with much out-of-date nomenclature...this reference has a large number of useful line drawings and photos (including 16 plates w/ 9 photos each = 144 photos at the end). The name of this work is rather misleading...as there's no "key" per se. Instead, the galls are organized by plant families of their hosts, listed there with (usually very) brief comments on rearing records, perhaps some minimal descriptive info, and abbreviated bibliographic references.