Book The Last Letter Deluxe edition — Rebecca Yarros

Beckett,
If you're reading this, well, you know the last-letter drill. You made it. I didn't. I need one thing from you: get out of the army and get to Telluride. My little sister Ella's there, raising the twins alone. If I'm gone, that means I can't be there for her. I can't help them through this. But you can. So I'm begging you, as my best friend, go take care of my sister, my family. Please don't make her go through it alone.

Ryan
When Beckett follows his best friend’s dying wish and arrives in the small mountain town of Telluride, he expects grief, guilt, and the weight of a promise he’s not sure he can keep.
What he doesn’t expect is Ella. Fiercely independent and determined to hold her family together, Ella is still reeling from loss after loss. But as Beckett slowly becomes part of her life, their shared grief begins to turn into something neither of them saw coming.