The most memorable designs for mythos creatures come from an understanding of the creatures anatomy. Once you have the musculo-skelatal structure nailed down it imbues the image with a depth of life on the page/screen. There are plenty of 'monsters' in prehistory (the Devonian period has some nasty looking examples) to use as a starting point, for a recent piece of artwork featuring a 'hunting horror' I began with fossils of the Dunkleosteus fused it with the Coelacanth and 'evolved' the end result by a few million years into a land creature. As has been mentioned before googling the base form of the creature I was looking for produced enough reference images and video for the final artwork.
I'm not sure about book titles, sorry. However I know where to get loads of examples, mostly from games. Final fantasy is a good one, more recent ones though I mean you could literally go to Google images and type 'final fantasy monsters'. Here's an example: [link]
World of warcraft is also an okay example, so again just search that + monsters in Google images. Hope this kinda helped and Goodluck