unfortunately, there really aren't any good, up-to-date books on Syngnathid care.
Scott Michaels has a decent section on them in Reef Fishes vol. 1, but that's about it.
there is a soft cover book called "Working Notes" available from seahorse.org which is a good primer on disease and treatment of SH.
Seahorses, Pipefish, and Their Relatives by Rudie Kuiter is a great book for ID purposes, but has no info regarding captive care.