To sign book, put your hands together, palm-to-palm. Then keeping your pinkies together, open up your hands as if opening up a book.
That book-opening motion is often difficult for babies, so you will see this approximated as the end of the sign with the hands apart. Or sometimes, it will just be the start of the sign, like the hands together.
The book sign is a great way to introduce reading time. This is a great fun sign, because babies love story time, and learning the sign is a great motivator to learn by reading.
A great way to introduce the book sign is to make the sign just before you read your baby her bedtime story. For many children, this is a favorite time of day, so they are highly motivated to learn the sign.