Having basically no artistic skills whatsoever, I use either
FatCow's or
FamFamFam's icon sets. Both free, both excellent.
Despite having used FamFamFam's for years for my coding projects, I'm slowly drifting towards FatCow's those days, mostly because it's still being updated (last time was in May), and it's got two eReader icons.
Both have a reasonable selection of open/closed book icons; I just fire The Gimp and derive more colored variations of the default blue. I end up with enough emblems for my uses (on device, WIP, Completed, Web original, Commercial, nonfiction, etc, etc.)