Across the Sea of Suns1984

From back cover Warner paperback March 2004:

2076: Lancer, Earth's first starship, is on a mission to find interstellar civilizations. Although astronaut Nigel Walmsley's experiences with alien encounters make him the expert, no one believes Nigel's theory that machines are the dominant intelligent beings left in the galaxy and that their purpose is to annihilate all organic life.

Then the explorers discover once-living planets where only machines remain -- and ruined worlds where fugitive survivors must evade omnipresent and lethally advanced A.I. warships. By the time the crew of Lancer realizes that Nigel is right, it will be too late -- for the machines have already found Lancer... and Earth.