The one which I had in mind was Frank Cottrell Boyce's Millions, about two lads who find loads of money, one of whom is obsessed with saints, who appear to him.
I'm not sure that autism is actually mentioned in the book, but I think it is a pretty clear depiction. Cottrell Boyce also wrote Framed more recently, which is also from the PoV of a young boy, this time less explicitly autistic, but with a recognisable set of behaviours.
Il Sotto Seme La Neva
