When you first start out on the server, you are given a limited amount of claim blocks; however, you earn more by playing. You get 100 per hour of un-afk play time. You can check your progress with this amount with /claimlist. You can also get more by donating or voting.
I want to make it clear to everyone in advance, when we update to 1.13.2 any books in booksheleves (the GUI) will be deleted with the plugin! We will not have time to deal with replacing stuff. So please move your books into chests before the update!