my absolute favorite book as a kid was trial by journal by kate kline, and as a tween it was the year of secret assignments, both of which are a combo of letters/emails/diary entries etc
I also liked speculative fiction/more grounded sci fi and fantasy:
-heir apparent by vivian vande velde, a vr middle school novel
-12 impossible things before breakfast by jane yolen, a series of creepy/fantastical short stories
-the true meaning of smekday by adam rex, a fun romp of an alien invasion novel from the perspective of a teenage girl
My number one rec is the book when you reach me by rebecca stead!! just a beautiful coming of age novel about friendship and love and also (unexpectedly) time travel. i haven’t reread most of the others in ages but I revisit this one every few years.
(Also possibly a controversial opinion, but if you never read twilight it might be worth it for the cultural experience lol)