I am an avid reader and may recommend books that I find helpful, or comment about places to find good deals on print books. If you read a lot, you probably know where to find cheap books online, but here are some places I use often.
Ebay.com can be a good source, as well as Half.com.
Of course, Amazon.com is an excellent source for cheap books.
I also find Google Books to be helpful with scanned books that are hard to find. Just go to the Google home page and click on the apps tab, and you should see an option for Books. Click on the books tab and a new search page will appear. You can search by author, subject, title, or even a certain word or phrase. I have been able to build a decent library of scanned books by saving the scans to my computer. You may find this to be helpful as well.
I discovered the website archive.org recently and have found many books available for download there.
I also enjoy thrift stores and antique malls. I have found some good items during my explorations while out and about!
If you have found other good sources for books online, let me know. I’d like to know about it.