Older book about team of time travellers operating outside light cone

I remember reading an older sci-fi book (maybe 50s-60s?) about a professional team of time travellers which operated outside the light cone to avoid breaking causality. Their purpose is to travel to remote planets and manipulate the society for their own ends.

The book covers one such mission, to a planet where the inhabitant live in trees and their architecture and furniture are fashioned out of the trees. Unfortunately I'm very hazy on the rest of the details. I remember the book's quality was very mediocre, the author definitely not one of the great writers, and there's a plot twist (which sci-fi story doesn't?) and the team doesn't make it out completely, some might have even stayed.