Carry On by Rainbow Rowell

Simon Snow is the "Chosen One," a powerful magician destined to save the magical world from a destructive force known as the Insidious Humdrum. He's attending Watford School of Magicks in his final year, but everything's a mess—his magic is glitchy, his mentor is missing, and worst of all, his longtime nemesis and roommate, Baz, hasn't even shown up.

When Baz finally returns, the two form an uneasy alliance to uncover the mystery behind Simon’s past and the Humdrum’s identity. Along the way, romantic feelings unexpectedly blossom between them, shifting their relationship from enemies to lovers.