Fatal Shadows by Josh Lanyon

One sunny morning Los Angeles bookseller and aspiring mystery author Adrien English opens his front door to murder. His old high school buddy (and employee) has been found stabbed to death in a back alley following a loud and very public argument with Adrien the previous evening.

Naturally the cops want to ask Adrien a few questions; they are none too impressed with his answers, and when a few hours later someone breaks into Adrien's shop and ransacks it, the law is inclined to think Adrien is trying to divert suspicion from himself.
Adrien knows better. Adrien knows he is next on the killer's list.