Pre-read books at greatly reduced prices.
If this is a non-profit organization you can send an email to all the radio and TV stations in your area and ask them to do a PSA (public service announcement). Write it up as "A pre-read book sale will be held on Saturday (date) from (time) to (time). All procedes to benefit (name) ...
Also in my area our cable provider has a channel that you can send in a simple ad and they put them up as slides under community announcements a couple of times a day. Also contact the Chamber of Commerence, and city to see if they have a community events page on their websites.
As long as you are a non-profit organization you can get a lot of free advertising.