Poseidon’s Steed is all about seahorses: their lives, the art they have inspired, the misadventures of the Lydian gold treasure (yes, there was a seahorse brooch in there — echoes of Loot), their use in Chinese medicine use and their place in Pokemon figures. A very wide field.
The author, a marine biologist and scuba diver, is at her best when describing the strange lives of these fishes (yes, fishes — and I thought they were crustaceans!) and she includes delightful drawings of seahorses from old books. I found the non-biological stories much less interesting and often meandering far from the critters that inspired them.