Search the Sky - Frederik Pohl & C. M. Kornbluth

Ross, a trader from a declining planet, is recruited to journey to distant human colonies to discover why they’ve lost contact and appear to be collapsing. He travels via a secretly held faster-than-light ship, visiting planets whose isolated societies have warped in various extreme ways. Along the way he collects companions, confronts repressive cultural regimes, and uncovers that genetic drift and isolation have doomed many of the colonies. In the end he reaches Earth, realizes the broader decay stems from separation, and resolves to reunite humanity to prevent further collapse.