I don't know where else you could look but the common E-Book & Audiobook places. So far many Books were not listed in the usual piracy sites. I had to search explicitly for the only Book/Comic/Audio Book related ones.
These sites however have TONS of Material in all kinds of categories. So even if there wouldnt be a "Artbook" Filter, if you look for a reocurring comic, make use of the reocurring name and hit the search with it.
You may have to check what are the best sites for your language though.
You could also go the route of directly downloading from spotify with Zotify. If you got premium it downloads in full 320kps. (Make sure to use the
--download-real-time=True
toggle to raise no suspicion for spotify servers. This way you download as fast as if you were listening to the songs/audiobooks yourself. Its a command line tool. Though if it can remove possible DRM Watermarks from the audiostream - no clue. But i wouldnt think they actually do that.