Survival: The Star Quest Trilogy, Book 3

When a human team is sent to scout a few hundred light-years in front of the death wave, it encounters a civilization far in advance of our own, a civilization of machine intelligences.These sentient, intelligent machines have existed for eons and have survived earlier "death waves" - gamma-ray bursts from the core of the galaxy. They are totally self-sufficient, completely certain that the death wave cannot harm them, and utterly uninterested in helping to save other civilizations, whether organic or machine.

But now that the humans have discovered them, they refuse to allow them to leave their planet, reasoning that other humans will inevitably follow if they learn of their existence.