Hal Clement 
Close to Critical [EPUB ebook] 
Mesklinite Book 2

Shrouded in eternal gloom by its own thick atmosphere, Tenebra was a hostile planet: a place of crushing gravity, 370-degree temperatures, a constantly shifting crust and giant drifting raindrops. Uncompromising — yet there was life, intelligent life on Tenebra. For more than twenty years, Earth scientists had studied the natives from an orbiting laboratory… and had even found a way to train and educate a few of them.