I don’t own almost any of the books that I present. I have about 6.7 gigs of medical history texts/images, 10.5 gigs of natural history, and a ton of online sources.
The page is a bit outdated (especially the tumblr list), but I still use a lot of those sources, especially OpenLibrary.org and the Science Museum of London site.
For animal anatomy, it depends what you’re looking for.
I have always been a fan of zoological monographs, like this one detailing monotreme anatomy, but the collections of the Proceedings of the Zoological Society of London also have a lot of anatomical features.
For plants, Curtis’s Botanical probably has what you want.