I have a relatively large library of books with a considerable portion of it being educational texts. Everything from mathematics text books to learning to fly a plane is at my fingertips. This is a thread noting the best places to find informational books with which to expand you knowledge base.
Library book sales are a great place to find books on a budget. Most often what I find here are travel books several editions old, language books older history books and of course scientific texts. Most of the books being sold will be older but that does not necessarily mean their information is not valuable. Sometimes school libraries sell their old books as well as text books. That would all depend on your local districts.
Your local paperback/used book store. Unfortunately around my area out used book stores are not terribly great for finding information other than chicken soup for the soul and financial well being guides. Don't overlook them though. They are the exact type of place you will find a book filled with unique information such as learning to breed cats or any type of self help book you may ever need.
Barnes & Noble has current information on every subject as well as being pretty much the only large chain book store around. Unfortunately most of the information in the books is for general audiences and will mostly only get you on your path rather than becoming an expert. Again this isn't bad just be prepared to look a little harder once your informational needs become more specific.
My favorite publisher is Dover books. They have an ungodly amount of books on any subject imaginable. Seriously this is where I get most of my text books. They are also fantastically priced. You could learn college chem for under $50. You could learn complete history of a people for $20. Often the material is rather dense as it is written by experts of their respective fields but if you are serious about study these books are worth a shot.
Your local community college. Not their bookstore silly the bulletin boards. It never fails that a student will take a class buy a hundred dollar text book only to use 5 chapters and never open it again. Oftentimes students will bundle books for a semi-reasonable price and stick an ad on a bulletin board. Find what interests you and read on!
Garage sales! Once in a while I will go to a garage sale and see that they have a decent selection of books. Usually when this happens I will know if they are worth a look by what the people are selling. Anyone selling sporting goods or utilities or tools that are in good shape will usually have some sort of manual at least that could be of interest. It really never hurts to look. I got a first edition copy of an incredibly popular title for 50 cents. Its worth $200.
More to come and don't be afraid to share your own ideas for obtaining info on the cheap.