Dragon's Egg by Robert L. Forward

I was revisiting an old friend, for some bizarre reason I didn't own a copy so when I saw one sitting around minding its own business I pounced with glee. This is science fiction at it's best, detailed, informative and compelling. The kind of book where you resent the need to sleep because there are things going on you don't want to miss.

The story is based on a species evolving on the surface of a neutron star, they are tiny, high density and utterly alien. A human ship makes contact just as the Cheela reach a stage of development high enough to talk back but the time scale they live in means that a Cheela lifetime passes in 15 human minutes.