Albums, Annuals & Pocketboooks:
According to the Oxford English Dictionary, in American slang “pocket-book” – a form on which the album was modeled – also refers to “the female pudenda” (1993 addition, CD-Rom), a euphemism that (albeit) anachronistically sexualizes an album's textual body.  Writing in an album, then, could be seen as a transgressive and secretive act and one that the owner (usually a girl) was generally privileged to share with a famous author (male or female).