Directory of Open Access Books
Recent scholarly books, made freely available by their authors. Downloads mainly in PDF format.
HathiTrust
Major free collection of digitised books and journals. Good for older material.
Open Humanities Press
New titles, many focussed on digital culture and digital art.
Project Gutenberg
Older, out-of-copyright works on a wide range of subjects. Includes an art collection and an architecture collection which can be searched separately.