This isn't really within the spirit of this thread because it's basically just a scientific picture book, but I am absolutely
obsessed with this thick, pricy book I purchased earlier this year called
All the Mammals of the World. True to its title, it features an illustration of
every known mammal species (and many subspecies) as of the time of publication. Over 6500 illustrations. It's incredible just being able to flip through it and see
everything the class has to offer. Something new jumps out at me and sticks with me every time.
There's also a 9-volume version that features lengthy writeups as well, but that one's far more of a commitment, both price- and space-wise. It's on my mental wishlist, though.
There's also a
sister book that does the same for birds that I also own, but birds just don't strike me quite the same way as mammals. Still super worth it if you're even remotely into birds imo.