FriendshipBooks or FriendshipBooklets are usually abbreviated as FB's. They are small booklets of addresses of penpals with some general information about each one of them. They are passed from one penfriend to another getting filled with useful addresses along their journey from a country to a country. Many penpals swap them as they present valuable information about future penpals and swappers. For me, it's always amazing to find out where the FB's have traveled and who had signed them.
FriendshipBooks come in different sizes, shapes and forms. What stays constant is the organization of the first page/sheet. They are always made for someone and by someone (whether that be one and the same person or two different people). Also sometimes there are notes of why this FB was made, such as "via label bag", "via FB game" or even "sorry FB" (when the maker of the FB can't be penpals with the owner).
An interesting fact is that the words "FriendshipBooks" can be also used in regards to making books (notebooks) filled with neatly decorated pages about the friendship you have with a specific friend or sheets of decorated short intros of your friends (in such a way, it closely aligns with the word "AddressBook").
This blog will only focus its attention on the PenPal meaning of the words Friendship Book, that is as in FB's.