A Hero At The End Of the World by Erin Claiborne
Seventeen-year-old Ewan Mao knows one thing for certain: according to prophecy, it's his destiny to kill the evil tyrant whose dark reign has terrorized Britain for as long as he can remember. Although he's just a normal boy, deep down Ewan is confident that he has exactly what it takes to be a hero. But when Ewan's big moment comes and his best friend, the clever and talented Oliver Abrams, defeats the villain for him, Ewan's bright future crumbles before his eyes.
Five years later, while Ewan is living at home and working a minimum-wage job, Oliver has a job as an Unusual in the government's Serious Magical Crimes Agency, the life he and Ewan always dreamed of. A routine investigation leads him and his partner, Sophie Stuart, to uncover a dangerous and powerful cult... one that seems to have drawn his former best friend into a plot to end the world.
Note that there's both a "het and LGBT" romance and the book features "two protagonists of colour."
Interestingly, this novel was funded by a successful kickstarter campaign, designed to get three novels written by authors of fan fiction published. My thanks to blog reader Julie for the recommendation! Julie "enjoyed every second of this novel" and described it as "kind of Douglas Adams meets Neil Gaiman in a Harry Potter-esq story staring a queer protagonist (Ewan Mao)"
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