Baen, 1989. Mass market paperback. Novel about the discovery of a Dyson sphere-like artifact surrounding a star, first published in 1975. First in the trilogy, which also includes "Orbitsville Departure" (1983) and "Orbitsville Judgement" (1990).
Two hundred years ago mankind found Orbitsville, a sphere whose habitable inner surface comprised living space equivalent to five billion Earths. The resulting migration was enthusiastic - and nearly total. Earth is now a backwater place. This book continues the story begun in "Orbitsville".
Orbitsville - scene of two of Bob Shaw's novels - is a vast hollow world completely enclosing its sun and habitable across its entire inner surface. At the end of "Orbitsville Departure", the whole world was shifted to an alternate universe and this book tells what happens next.