After completing Horror from the Deep, you are rewarded with a damaged book of your choice: either Saradomin, Guthix, or Zamorak. You must complete your current damaged book before obtaining a new one of a different type from Jossik, and you can only buy one damaged book per day. Dropping or destroying a damaged book and talking to Jossik results in him returning it to you. You can check which pages are currently in your incomplete book with the "Check" option. Filling the book with all pages currently costs coins. Once you have filled the book, you cannot take the pages out.