The Cockatrice Boys - Joan Aiken
(found at Waldenbooks)
A British fantasy about an invasion of strange creatures, and the small army that fights them. I've seen it classified as "Young Adult" or "Adult Fiction"; the reading level is more "Young Adult." It's a pretty quick read, with a mix of legendary mythological beasts (cockatrices, manticores) and ones I believe Aiken made up (Flying Hammerheads, for instance). The message is either environmental or religious, or both.