The Idea Factory - The Battle of the Amnesiac
The book starts out at the scene of a big battle. The hero is standing in the rain, weapons in hand, staring down the main villain. They ask "what just happened?" The villain says "I just took all your memories." The hero escapes the fight but must recover what they lost and figure out who they were fighting and why. Moreover, are they the good guy or the bad guy and should they still be fighting.
As the book goes on, the main gets flashes of memories and thinks they're remembering their life. Only the memories aren't theirs. They were stolen from people that they killed. The main and the villain had once been partners until they had a falling out. They steal memories from their victims for espionage and to keep the few that get away from reporting them.