I am looking for a book I read as a young adult. It was from my dad's collection, and not a "young adult" book by any means. The story involves an attack on earth by a distant alien race who fire relativistic projectiles at earth. Earth then launches a ship with a small crew and a powerful AI system to retaliate. I don't remember much, but there was a power struggle between the captain and the AI. I also remember it got really, really weird about halfway through. Almost like a totally different book with lots of mystical stuff. I think there was something about a crow or raven. Like I said, really weird. I have no idea what it could be and it's driving me crazy.
Sounds like Catchworld by Chris Boyce
A fleet of bussard ramships is sent out from Earth to find the source of an attack. Early in their mission there is a power struggle between the captain and the ship's AI, just as described above. Unknown to the crew is that they are also part of an experiment where their nervous systems become integrated with the starship's AI. By the time they reach the enemy world they are all fully integrated within the AI with the exception of one crewmember who was immune to the drugs used on his crewmates.
At the enemy world it is found that the source of the attacks is a being called The Crow