Dragon's egg (1980)

Forward imagines intelligent life on a neutron star, where civilisation develops at a radically different pace from the humans observing it. The tiny scale of the cheela's bodies sits beside a history that can outrun their visitors' expectations. That mismatch makes the book's wonder specific rather than merely enormous. Choose it for detailed speculative physics and biology, with an appetite for explanation: the imagined environment changes what being a neighbouring civilisation can mean.