How do I authorize my device from the store menu to download books from the bookstore?

My phone and MacBook are under the same apple account and are synced. However, when I try to open a book on my MacBook, I get an alert saying "You must authorize this computer from the Store menu before you can download previous purchases." I have no idea how to authorize my computer. I also don't understand why this is the case when I can open all my other books on my iPhone and MacBook.