David Levithan has traversed the giddy highs and dismal lows of young love — gay and straight — for a decade in his fiction (“Boy Meets Boy,” “The Lover’s Dictionary”) and collaborations (“Nick & Norah’s Infinite Playlist” with Rachel Cohn; “Will Grayson, Will Grayson” with John Green). In matters of the heart, teens have certainly taken him to theirs. Levithan’s new YA novel proves to be his most nuanced yet, a tender meditation on identity and romantic love that evolves with surprising grace from a rather odd premise. Each morning for the past 16 years, A, a kind of drifting, non-corporeal consciousness, has awakened in the body of a different person, an existence that A finds both lonely and “remarkably freeing.” While in the body of self-absorbed Justin, though, A becomes smitten with the boy’s gentle, uncertain girlfriend, Rhiannon. Most of the plot revolves around A trying to spend time with her, whether in the body of a rocker, a geek, a pretty cheerleader, a burly football player or a scruffy, heavy-metal fan. A’s wry, wistful voice keeps all this from devolving into “Freaky Friday” shenanigans, as do the sensitive glimpses into the lives of very different people, especially a suicidal girl and a confident transgendered teen. A subplot involving one of the boys determined to expose the “demon” that possessed him seems less developed and compelling than A’s hopeful search for true connection.