The Book of Memories is major plot device in Silent Hill: Book of Memories.
The protagonist receives it on their birthday from an unknown sender. Howard Blackwood, the local mailman, assures the player that the book was meant for them before he departs. The protagonist opens the book and finds that all of their memories are written in it. Curious, the protagonist decides to change some memories before falling asleep. When they awaken, they find themselves in an Otherworld-like dungeon.
In actuality, there are multiple Books of Memories. These Books imbue the writer with God's will. Vincent Smith was researching them, and discovered Dahlia Gillespie sent them to Wish House Orphanage.
It is also implied that the main canon universe Michelle Valdez finds her own Book of Memories and realizes its power to change reality.