The reader knows who Smith and Mapplethorpe will become, so it is intriguing to read about his continued attempts to encourage her to become a musician, while she urges him to delve into photography.

“Just Kids” is the most spellbinding and diverting portrait of funky-but-chic New York in the late ’60s and early ’70s that any alumnus has committed to print. The tone is at once flinty and hilarious, which figures: she’s always been both tough and funny, two real saving graces in an artist this prone to excess. What’s sure to make her account a cornucopia for cultural historians, however, is that the atmosphere, personalities and mores of the time are so astutely observed.

It’s possible to come away from “Just Kids” with an intact image of the title’s childlike kindred spirits who listened to Tim Hardin’s delicate love songs, wondered if they could afford the extra 10 cents for chocolate milk and treasured each geode, tambourine or silver skull they shared, never wanting what they couldn’t have or unduly caring what the future might bring. If it sometimes sounds like a fairy tale, it also conveys a heartbreakingly clear idea of why Ms. Smith is entitled to tell one.