Every Day is a 2012 spec fic young adult novel by David Levithan. It tells the story of "A", a genderless being who wakes up each day in the body of a new random person in the vicinity of the same age. At the time of the story, this person is 16. A doesn't know what they are or why this is happening. This is the only life they've known, and didn't even realize it was abnormal until sometime around age seven when they mentioned it to the parents-of-the-day.
The hosts are always within a few dozen miles or so of the previous host, and as such they generally travel stay within the same town/region (the exception being if the host happens to have a big trip happening day-of possession), and they end up crossing paths with past hosts throughout the story. While possessed, the host has no agency or awareness, and A has access to their memories. A former host will remember nothing particular, though A can intentionally create false memories to leave behind for them the next day.
A is not malicious or abusive to their hosts. They have a rule that they do their best not to mess anything up with their hosts lives. This is put to the test when A falls in love a host's girlfriend and seeks her out in subsequent days with the desperation of a 16-year-old in love. This is half the plot itself — it's a teenage love story disguised as spec fic!
Okay, but there's also more to the plot. What is A? We don't really finds out, but we get some intrigue going on there. I'll go into it below, but be warned Spoilers follow
Spoiler Section
Okay, so A loves Rhiannon and meets up with her in subsequent days. Tricky bit: A is in a different body each day. Even once Rhiannon learns A's secret, it's weird that A is in a different body, and a different gender, each day, and that these bodies have lives — boyfriends, girlfriends, families.
A has that rule about not mucking with their hosts' lives, and tries to not cause too much turmoil with their visits to Rhiannon, but early on, one good, milquetoast conservative Christian boy ends up driving to a party and doesn't make it back before midnight -- when A vanishes and the body invariably returns to the host. He's found asleep in his car, and vehemently denies having taken the car on a joyride. He makes noise, and eventually finds fame with local media seeking others who have been possessed by the same demon as he was.
Eventually a local pastor gets involved and when A finally decides to meet with them, discovers that the pastor is himself an "A"-like creature. This pastor, however, is older and has a way to stay in the same body. He offers A guidance on how to do the same, but A recognizes him as a bad guy and flees. It's somewhat unsatisfying, as I would like answers from this guy.
End Spoiler Section, except some very minor stuff
David Levithan is know for queer literature, and this one is that. A doesn't identify as a boy or a girl. I listened to the audiobook and thought of A as a girl due to the female narrator, while my friend thought of A as a boy because the first host we see is a boy. A also has at least one gay host and one trans host.
The book doesn't focus on the diversity aspect, but instead just has it, and it's done well. Different genders, races, and classes. Each family dynamic is unique and presented well.
I think the two biggest turn-offs in this book is the degree of teenage love, and the fact that A totally stalks Rhiannon, and mucks with the hosts' lives because of said "love". Deep, overwhelming "I met you yesterday" teenage love.
This is a very good book if you like teen romance, and a good book if you don't hate teen romance. There's a sequel of sorts, Rhiannon wherein Rhiannon tells the story from her perspective, and a prequel, Six Earlier Days which tells of some days earlier in A's life. There's also apparently a 2018 film adaptation directed by Michael Sucsy.
If you liked this, or if didn't but think the concept is interesting, my best recommendation is The Host by Stephenie Meyer or The 7 1/2 Deaths of Evelyn Hardcastle. If the unusual narrator aspect appeals to you those are just that, and I'd also suggest Ancillary Justice by Ann Leckie.
If you want annoying teen romance, maybe try Will Grayson, Will Grayson, also by David Levithan, paired with John Green.