Backing up my iPad

I hav purchased an extra 50gb of I cloud for my Ipad...but I only want to use it for extra storage on my Ipad..I do not want to share with other Pc's....Am not sure how to go about this...If I just click on back up will it back up everything on my Ipad...my problem with this is that I have audio books and they cannot be redownloaded thru Icloud.....Is it possible to back up without backing up my audio books...Also when I do back up...am I able to redownlooad more than once

Would appreciate any advice....Many Thanks