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The Spheres of Heaven by Charles Sheffield
This book follows Mind Pool, although it is not a direct sequel. Humans are sequestered within a light-year of Earth by the three other intelligences in the galaxy because of their violence. The other intelligences control the Links (wormhole-like handwavium) that allow travel and suddenly there is a new link where none has been before. Each intelligence sent a ship that didn't return (and humans did, too, secretly). Now they want to mount a formal mission, run by humans. When the humans go through the link, they find a strange place. The other ships are there with their crews, but there is a hostile, also-foreign, alien force on the planet. The story of the multiverses, the depiction of how different species think and act, the adventure and tension, and the plot are all very good. The book ends sufficiently, but it's clear there was to be another book and Sheffield didn't get to write it before he died. Great adventure and characterization.