you could just look up mythology online... stay away from the .com URL's when possible... they aren't regulated the same way as other sites.
use multiple sites to augment your research as well as any good books you can find. I suggest this because, like words in any language, the names of mythological beings are subject to having multiple different meanings... depending on where you look, a siren could be multiple different beasts. I don't know, it has helped me in the past.