Star Ways by Poul Anderson
Most of the main characters in the book are "nomads" - people who live on starships who identify with their ship rather than a home planet. And whose starships only engage in acting as traders to the extent it's necessary to do something to acquire supplies for themselves. The captain of one nomad ship, Jacquin, has concluded from the disappearance of some ships and other clues that there is an unknown civilization on the fringe of human space which poses a possible threat. About the same time Jacquin arranges to have his ship go to investigate, Earth's authorities have drawn a similar conclusion about an unknown civilization. Earth sends one of their agents, Trevelian, to find out more. Although the nomads don't like the Earth authorities, Trevelian manages to have Jacquin take him along. Together they start to investigate.
One of the crew develops a romantic relationship with an alien. Trevelian develops a romantic relationship with one of the nomad women.
The unknown civilization has been prepared for the coming of investigators or explorers. The aliens are humanoid, but have a world where all the life forms cooperate with each other. On the one hand, it is indicated that although the aliens lack a machine tech science, they are knowledgeable about biology. That might indicate that the aliens may have engineered the cooperation among life forms, rather than cooperation being the nature of life on their planet prior to the evolution of intelligence. On the other hand, that would seem to suggest that all life on the planet is now domesticated / bred species, but it is never stated they are. Nor am I sure that would be consistent with the alien's philosophy. In any case, it's somewhat like some Gaean-type visions of planets of life acting as one happy family or a democratic union.
The aliens maneuver the nomads to the aliens’ planet and capture them. The aliens intend to either convert the nomads into agents acting for the aliens or keep the nomads on the aliens’ planet out of contact with other humans. The nomads are outnumbered - as well as having the rest of the planet's life forms ready to confine the nomads. And the nomads' spacecraft are not readily available. Yet, most of the nomads don't want to stay or be someone else's agents.
Story endings are always tricky. However, the ending of this book seemed more abrupt and begging for further answers than is the average ending.
Some readers may like the idea of getting a view of a Gaean-like world and civilization. I’m not sure whether these readers will be entirely satisfied. This is not an entire book of exploring all the various aspects of such a planet. It may be more like an appetizer than an entrée.