Marine Invertebrates 500+Essential-to-Know Aquarium Species by Ronald L. Shimek, Ph.D. has been very helpful to me in identifying stuff I see at the LFS. It is a pocketbook size soft cover published by Microcosm and TFH and it's pretty inexpensive (under $20.00 US, if I remember correctly). It has good photos and concise information. Each animal entry has sections on Maximum Size, Range, Minimum Aquarium Size, Lighting, Foods and Feeding, Aquarium Suitability/Reef Aquarium Compatibility, and Captive Care. The information is brief, but complete and detailed enough for you to decide at the LFS if you should pass on that Percnon gibbesi Sally Lightfoot crab you like so much in the tank in front of you.