Shipstar

Excellent world(s) building including fascinating and well-developed non-human races and awe-inspiring physics. And the human side of the story was great, too!

This finishes the two-part story. (Read #1 first.)
Benford and Niven are champions at building humongously big alien structures. Unlike the Ringworld, which is a big dumb object, the Shipstar is a big smart object, with layers of intelligence. (You'll have to read it to find out what I mean by that.