Finders Keepers

The second standalone novel in Stephen King's Bill Hodges trilogy (Mr Mercedes, Finders Keepers, End of Watch) - and the basis of the third season of Mr Mercedes, an AT&T Audience Original Series (10 September 2019).
1978:
Morris Bellamy is a reader so obsessed by America's iconic author John Rothstein that he is prepared to kill for a trove of notebooks containing at least one more unpublished novel.
2009:
Pete Saubers, a boy whose father was brutally injured by a stolen Mercedes, discovers a buried trunk containing cash and Rothstein's notebooks.
2014:
After thirty-five years in prison, Morris is up for parole. And he's hell-bent on recovering his treasure.
Now it's up to retired detective Bill Hodges - running an investigative company called 'Finders Keepers' - to rescue Pete from an ever-more deranged and vengeful Morris...
Not since Misery has King written with such visceral power about a reader with such a dangerous obsession. Finders Keepers is spectacular suspense, and it is King writing about how literature shapes a life for good, for bad, for ever.

Stephen King