Google would not be my first choice! The problem with Google is that a lot of the articles that turn up are unedited or written by people who don't have a clue.
If you're looking for something specifically on the things in your tropical marine aquarium, then 'Reef Invertebrates: An Essential Guide to Selection, Care and Compatibility' is one of the better books around.
But do be aware that identifying small creatures like isopods and annelid worms is not something that is easy to do. I have a zoology degree and worked as a marine biologist for a while, and even identifying the stuff from the Atlantic that I am familiar with is challenging. For someone without a degree in zoology, simply identifying things to class-level will be an achievement in some cases!
If you merely want to know something about what these sorts of animals are and how they live, then a more general invertebrate zoology text book will probably be more useful. I happen to love 'Animals Without Backbones', a book that has been reprinted and updated many times since 1938. It's a lovely, readable primer to these animals.
Cheers, Neale