The best way to find individual e-books is through LibrarySearch; to filter your search, use the 'Collection' filter on the right and click e-book (you may have to click More options first).
You may find these useful:
- APA E-books Collection: psychology textbooks published by the American Psychological Association in 2011 and 2012
- Oxford Scholarship Online (from Oxford University Press): collection of psychology e-books
- Oxford Handbooks Online: thorough introduction to the current thinking within the subject of psychology