In the UK, we can buy 'a book of stamps' to use when we post letters. A 'book of stamps' is a small piece of card with new stamps inside. We can buy a book of 6 stamps, or a book of 12 stamps.

In the US, years ago, you could buy stamps in a package that looked kind of like a small book. There would be four stamps on the first page, four stamps on the next, etc. Every time you sent a letter, you would use one of the stamps in the book. (I'm having a hard time finding a picture). Today, when you buy multiple stamps they come on a sheet but we still refer to it as a "book of stamps."