I thought I’d just toss it out there that Ken Tennessen’s Dragonfly Nymphs of North America: An Identification Guide will be available in April from Springer and other online sources. This is not a field guide, but rather a richly-illustrated lab manual and desk reference. It isn’t cheap, but it will be the gold standard for a long time to come—I know because I proofed the draft. Twice.
By the way, my only relationship is as a friend of the author. I’m not getting a cut.