YA fantasy where Jacob goes to his aunt's house and falls through a portal of some kind, maybe a mirror, and lands in an alternate reality. There are fea and dragons and gnomes. The book has a green cover and I read it about 9 years ago. around 2012 or so.

[Kora]

Jacob, about 15 years old, goes to his aunt's house because I think his parents are having a divorce. He is moody and full of hate. He falls through a mirror/ portal to another world. There are fea, dragons, gnomes, and all other mythical creatures.SLIGHT SPOILER: At one point Jacob is in the land of the fea and they offer him food. He is about to take it (making him be enslaved to them) when I think the love interest saves him. At the end of the book, Jacob has to go back to his world, but his love interest (I'm not for sure, but I think elf. Tall, thin, blonde almost white hair) has to stay in the world of the fea. It is about 300 pages most likely.
The cover had green smoke swirling around Jacob and a mirror/ portal. There are green thorny vines on the corners.

[Kris]

A guess - The Revenge of the Shadow King by Derek Benz? (parents divorcing, grandmother's house, portal)

[Jamie]

Sounds like Reckless, by Cornelia Funke. Book 1 in the Mirrorworld series.