Illustrations

300 DPI if it goes to print, other things depend on the size of the book first and foremost.

I was also considering making use of illustrations for a story. Does anyone know of guides I could look at for finding a suitable illustrator?

I say as an artist myself: go bigger than necessary.

You can always scale down a copy easily (never touch the original). Scaling up loses quality and you don't want to have to do that. As for working size, I would say 10 x 15 at 300 dpi if you are working with a 1:1.5 ratio. If doing line art (straight black and white), go with at least 600 dpi to avoid anti-aliasing blurriness.