From Fanlore, the Fan-run wiki about fandom:

Tracing The Path of Brian’s Friend Book.

“"Friend Books" were used by pre-teens and teens in the 1970s and 1980s as a way of finding other like-minded fans in the pre-Internet era.   Unlike the dreaded “slam books” which began circulating in the US in the 1940s, these books were not used to bully,^[1][2][3] but as a means of connecting with other fans.

Some “friend books” were sized small enough to fit into an international envelope (2-3 inches), and consisted of no more than a few pages. The covers were often pages cut from magazines or advertisements and were stapled or taped into a booklet shape. Many were multi-fandom, but some focused on single fandoms like Star Wars.

Fans would write their name and address, and list their interests. The book would then passed along to the next fan. When the booklet was filled it was to be returned by mail to the original fan. Often times questions were asked (ex: Who is your favorite Star Wars character?)