The bookstore clerk who let you read half a book on the floor without buying it was giving you more than a book.

Somebody decided you mattered more than the inventory. Letting a kid sit on the floor with an unpaid-for book is a small loss of a sale and a large grant of permission, and permission is the part that stays. What you learned was that a room full of books would let you in without proving anything first.

Go back and buy something, even cheap, from the place that did that for you. If the shop has gone, do the equivalent somewhere else: spend money at a counter where a person is standing. Then pass the permission on by telling somebody younger that it is allowed.