Why would you buy a book of prayers written by other people? Would you buy a script written by someone else to talk to your father on the phone? When we pray we should speak from the heart.
Although I am an evangelical Christian, I do have respect for the Anglican Book of Common Prayer. Catholics often use Missals for devotional prayer as well. If such books help the believer focus his prayers, they serve a useful purpose. The problem with such books, however, is that they can lead to “vain repetition” instead of sincere conversation with God.