Here are my favorite sources for free art-related ebooks. If you know of others, please email the editor to share them.
Internet Archive (main ebook page and search results for Art as subject) – the subject terms and search function here don’t work as perfectly as I’d like, but there’s gold for those who keep trying new searches)
The next two sources are less art history and criticism, more technique and how-to, and you’ll find overlap from the sources above:
100 Best Free Art Books from Gold Coast Art Classes
Free Artist Handbooks from The Color of Art