A window book is a book about something very different from your life while a mirror book is similar to your life. I prefer window books because I like fantasy.
I am reading The Sea of Monsters. Right now my book is a window. At the beginning it was a mirror because it was about a boy in middle school around my age. The story rapidly turns to fantasy after the beginning. After about 20 pages the kid goes to a camp for half bloods and goes on a quest to find the Golden Fleece and his friend with his friends.