Water Gardener's Handbook by Philip Swindells (Overlook: $19.95) dives with zest into the one area of home gardening still relatively unexplored. If you hadn't thought about adding aquatic plants to your horticultural repertory, this introduction might well tempt you to give them a try. As eight pages of color photos indicate, there is a vast range from which to choose, and the challenge is worth it. Swindells begins with the basics, including sections on construction as well as care. His appendices of available aquatic plants are among the most complete, with a list of suppliers especially valuable.