You might think it isn't worth reading a book when you know how it will end. We're so used to twist endings and cliff hangers that the perception now is that you just can skip to the end and figure out the basic story. For some books that might be true: if you know the end then the rest of the book is ruined because it was all just set up for that one big reveal. But for others, the journey from the beginning to end is the entire point (think The Hobbit).
If you can't tell by the title, They Both Die at the End by Adam Silvera is a journey book. Mateo and Rufus live in a New York City where a company lets you know the day you will die. Both teens get a phone call after midnight telling them today is that day. They aren't told how; just that they will die at some point that day. It could be in a minute. It could be in hours. The two meet on an app that lets you find a Last Day Friend. Someone to connect with and help you on your End Day. The story is how they spend their End Day. Mateo is afraid of the world and Rufus is mad at the world. But they click. Both know they'll never have what they dreamed of, but they have one day to live. So they're gonna live. Even if it kills them.
It's an interesting premise that'll make you wonder how you'd spend your last day and if you'd even want to know or if that makes it worse (I'm on the fence about this). I won't say it's not a bit morbid. I mean it is about death. But strangely it's full of a good deal of hope, and the belief that you can become who you've always wanted to be even if it's just for a short time.