I've purchased Dover Publication artwork for years, and especially like their colouring books.
Dover offer free samples of alot of their books, be it artwork, or colouring books, and I'm sure a lot of other crafters/colourist would like them. So I've collated some links for visitors to check out on Dover Publications of free samples, you can sign up to get updates yourself.
Here's a link to the free samples on Dover Publications website -
Fanciful Faces Samples
For those that love scary, but fun designs to colour
Day of the Dead Samples
Bizarro Land Samples