Some YA:

More Than This by Patrick Ness is beautiful and remains a favourite to this day. It begins with the main character dying and then waking up alone, and is carefully intertwined with flashbacks to his secretive relationship with his friend. Come to think of it, Ness - who’s gay himself - has a few characters in his novels who just ‘happen’ to be gay, which I love.

Grasshopper Jungle by Andrew Smith is quite graphic, both violently and sexually, and is quite hard to swallow for some, but when I was 14 it was my most favourite book. It is narrated by a boy who’s both in love with his girlfriend and in love with his (male) best friend, who is gay. He’s not sure. He’s trying to understand his sexuality whilst six foot tall praying mantises attack his town. It’s wonderful.

Additionally:

A Little Life by Hanya Yanagihara is hated by some but adored by others. 3/4 of the main characters are gay and many side characters ‘happen’ to be LGBTQ+. It’s an extremely dark and graphic book and perhaps some dislike it because they think it portrays gay relationships negatively, but I think they’re fully fleshed out and intensely real and vibrant.