
Ooh, thank you for linking it, Rainbowheart! I totally forgot to.

"Seven Types of Ambiguity" by Elliot Perlman is over six hundred pages.

This is a really, really long shot, but could it be part of the Circle of Three book series by Isobel Byrd? Book 14, The Challenge Box, has a girl who learns about her family history through her sister, who lives in or travels to New Orleans a lot. The girl learns some voodoo.

"The Weight of Feathers" by Anna Marie McLemore is one I thought of. I'll think of more.

The spinoff series from the Babysitters Club, the Little Sisters series, has a book with this plot, and a blond protagonist named Karen. The book is #56, Karen Ice Skates.

"Family Plot" by Cherie Priest has a haunted house that reveals itself to be conscious, as it talks to one of the construction workers and helps her with a mystery.

This might be a long shot, but could it be "Mandy" by Julie Andrews? It was written in the 70s. No nuns, but the girls go to church a lot, and Mandy is carried back by a man in a black coat.

Could it be "Beneath A Meth Moon" by Jacqueline Woodson?

Could it be "Maybe one day" by Melissa Kantor?

Following up to my earlier comment: the book I liked that was clearly a love letter to the documentary "Paris is Burning," is called "The House of Impossible Beauties" by Joseph Cassara. The book I firmly believe that ripped off Swimfan, Fatal Attraction, and others was "The Drowning Girls" by Paula DeBoard Treick, and I hated it.

Sometimes they're clearly love letters to the original. I can't remember the title or author, but there's a book I gave a glowing review to because it was a "Paris is Burning" based novel. Not quite a fanfiction, but it was clear the author had seen and enjoyed the documentary. Sometimes that is very clearly not the case. The harshest review I ever left was a one-star review with the sole sentence, "Clearly the author and I have the same favorite movie." She ripped the movie and several others I recognized off. I don't remember the name or author at the moment, either, but she ripped off Swimfan, Fatal Attraction, and several similar movies. I was furious that she thought readers wouldn't notice.

"If you could be mine" by Sara Farizan hits all the landmarks of the subject line.

This sounds like "Marvin Redpost: Kidnapped at Birth?"by Louis Sachar.

Could it be "Black and Blue" by Anna Quindlan?

RL Stine's "Truth or Dare"

"Every heart a doorway" by Seanan McGuire is the first novella in a series that fits this.

Unhealthy love obsession: Seven Types of Ambiguity by Elliot Perlman. His short story anthology The Reasons I won't be coming has plenty of them too, and a few doomed-love ones.

"The Boss" series by Abigail Barnette has a H who is 20 years older than the h.

Very much so. I'm glad for it. It can be a huge letdown to realize just how horrible some male protagonists I admired are, when I go back to read old favorites sometimes, but it makes me think on past behaviors too.