Prayer books can be wielded as if it were a shield to those in the Prayer skill. These will, in the beginning, add +5 to your prayer bonuses. You’ll have to finish the Horror from the Deep quest first though, and be a member instead of a free player. Here’s what prayer books can and cannot do for you.
Each of these prayer books will come to you “missing” four pages that you can add to it from things like player trades and finishing Treasure Trails. After you’ve completed one book fully you can go to Jossik (find him at the north lighthouse of the Barbarian Outpost) to get another. You can see him if you loss a book too and get another. After finishing a book, if your Prayer is up to 50, you can bless symbols of the Gods.