One of my top 5 favorite SF novels. Also a rare one one that I periodically re-read, and enjoy every time.

Loved the concept of "zones". Loved how it answers why Old Earth was never visited by star-faring aliens, despite how ubiquitous they are.

Loved the "Tines". One of the most interesting and inventive species I've read. Loved how their biology realistically affected so much in their culture, language, architecture, etc. Very well thought out.

Strong characters. Fabulous plot-devices and world-building, revealed in a natural way. Classic plot (it's really a Holy Grail variant). If I could write one SF novel as well-crafted as this, and no more, I would die happy.

But bonus: The sequels and prequel were good reads too, IMHO.