Wasn't able to track down the first book in this sequence, but it's pretty seamless to jump in on this second one. Worldbuilding is smooth and convincing, to the point that you almost might miss the fact that a lot of the stuff referenced actually happened in a second book, and isn't just stage dressing.
Antsy's inscrutably cool Swedish classmate shares that he's going to die in six months. Antsy offers him a month of his own life out of sympathy. Then everyone else wants to get in on the good deed. Things spiral. Life, (the fear of) death, and comedy abound Lot of heavy themes explored with a light, funny touch. The mid-teens boy perspective is solid, by turns insightful and hilarious, and helps make the content feel approachable instead of heavy. Faith/prayer/religion, parents' career and relationship conflict, and illness and death covered.
Mild dating content (kissing) but I'd say this is an accessible read for male and female readers from about mid-Elementary school age on up. Main cast are in the 15-16yo range, but tonally it feels more MG.