What is Every Day by David Levithan about?

Every Day is a young adult novel by David Levithan that was published in 2012. Straddling the genres of romance and fantasy, Every Day follows a rather unconventional protagonist who runs into some unique relationship obstacles.
The main character is known only as A, and A wakes up each day in a different body. A has no set gender or race, and their consciousness or soul inhabits the body of random people by no choice of A's own; they exist in one host body only for a 24-hour period. This proves at once fascinating and challenging for A, who is able to experience life from a stunning variety of perspectives while having no one stable, corporeal identity of their own. They have to accept their strange existence, and they make a set of rules for themselves: Don't get too attached to any one body or the people in that person's life. Respect that person's life by not upsetting the person's routines or behaving abnormally. Unfortunately, this means that A is rarely able to truly be themselves.
One day, things change: A wakes up in the body of a 16-year-old boy named Justin and falls in love with Justin's girlfriend, Rhiannon. Rhiannon has hit a rough patch in her relationship with the real Justin, and she and A bond over a day at the beach. While A is able to convince Rhiannon that A is a unique entity, no matter what form they inhabit, this makes developing a romantic relationship challenging. Throughout the course of the novel, A approaches Rhiannon in a number of different bodies, seeking to maintain their connection even as this means breaking their own rules, using these bodies for their own self expression for the first time.
Along the way, A comes to know themselves better, but not without conflict. While in the body of another teenager, Nathan Daldry, A behaves out of character for Nathan. Nathan—a shy, reserved bookworm—would never go to a party, but A does just that while in his body in order to see Rhiannon. When the 24-hour period is up, Nathan wakes up in his own body, convinced that he has been possessed by the devil. Angry, he decides to track down A, and when he spreads his story, he's no longer the only one looking for A.
While Every Day subverts the typical young adult romance in a fantastical way, at its core the novel is still very much a coming-of-age story. A and the people whose lives they touch learn a lot about living for the moment, discovering oneself, and finding love and friendship in unexpected places. Due to its popularity, Every Day was adapted into a film by the same name in 2018.