1. Where She Went
Let's get the most obvious out of the way first: if for some reason you've read If I Stay but not its sequel Where She Went, get on that. This book is just as (if not more) emotional and powerful as the first as the story of Adam and Mia continues, this time told from Adam's POV.
2. Looking for Alaska
This book is similarly centered around life and death in a very big way as it explores life, death, suffering, and all those big-ticket items. I'm pretty sure you've all read it already.
3. The Fault in Our Stars
Oh, another John Green book! And another book that considers big-ticket themes in a big way. As with If I Stay there's a sense of loss from the very beginning and while Hazel has no choice in her life/death, the way she handles her disease and her future death in regards to those she loves reminds me a lot of Mia in If I Stay and the struggles she has to deal with.
4. Adios, Nirvana
This is one of the biggest read-alikes for If I Stay, in my opinion. It's different from other books on this list because music is such a huge part of the story -- just as it is in If I Stay. It's similarly beautiful and heartbreaking. I recommend you read it.
5. Before I Fall
Alright, this is another huge read-alike. The main character is dead, or almost, or something like that. She's a bit in-between, same as Mia, and even at this late stage in her life she's trying to make the best choices possible. Beautiful. Powerful. Heartbreaking. Amazing. so many feelings.
6. I Now Pronounce You Someone Else
I realize that this one, a book about a teenage girl planning her wedding, is notably different from the others on the list. But there's a feeling to If I Stay that, to a certain degree, I Now Pronounce You Someone Else also shares. Both Mia and Bronwen are so in love and both of them are hugely influenced by family. Also, the writing is just great.