The "stroke" in "Stroke Book" is the frequent use of such periodicals to achieve sexual fantasy, arousal, and usually solo sex, that is, manual stimulation or masturbation to orgasm on the reader's part. The focus of such magazines is usually to highlight nudes of one specific gender in the pictorials, not both, and the perspective of one gender as narrator of a graphic erotic encounter in the short fiction.
"Hey, Sis, can I borrow your copy of nudie magazine? I need a stroke book, if you know what I mean."
"Okay, Leslie, but bring it back afterwards and don't get the pages stuck together!"