One of the most effective charity-type sales you can have is a book sale. People often have boxes upon boxes of books they don't want lying around in their basements, and are more than happy to give them away. Plus, people from all demographics are happy to come to a book sale, as they can probably find something that will suit their fancy. If you're trying to raise money, having a book sale for charity is a great way to go about doing it.
Collect donations of books by making some calls to your friends and family, especially anyone who is in a book club. Ask them to put together any books they don't need anymore, and have a central drop-off point near where you plan to sell them (you don't want to be hauling them around too far, as books are very heavy!). Ask local libraries and schools if you can put out a box somewhere on the premises for donations; you'll usually get a positive response if you explain what the deal is.
Price the books consistently, so that you don't have to mark each one separately (that would be way too much work for the extra few dollars you might get out of pricing a book or two higher). Decide on a price for each category you divide the books into -- whether you're just general in saying paperbacks vs. hardbacks, or if you get more specific and make categories such as "romance paperbacks" and "grade school textbooks."
Organize the books so that it's easy for people to sort through them and shop. Use boxes and shelves to keep them in order, and try to come up with categories or methods of organization that aren't too confusing for you or for other people. You want shoppers to be able to figure out where they should look for a particular book, should they come with one in mind.
Put on another inviting event as part of your sale, something that's related to books. Maybe you can convince a local author to spend the day signing his or her book at a table at the sale, or perhaps you can have an open mic where shoppers and local writers can bring by their works to read as background noise.
Find high-profile places to advertise, such as libraries and book stores that allow postings. Try schools as well, and community centers, especially near flyers about readings or other book-related events. Ask local websites to advertise as well!
You can also advertise on GarageSaleCow.com! It's free and easy to use.