Solaris by Stanislaw Lem. A psychologist is sent to a station in orbit around a planet covered by a sentient ocean to determine if research on it should continue. Lem’s work consistently blows my mind, actually–other favorites are His Master’s Voice (memoirs of a mathematician working on a project to decipher what might be a message from extraterrestrials) and Peace on Earth (an adventurer returns from a trip to the moon with his brain bisected, and the half that remembers what happened is both unable and unwilling to communicate it).
To be pedantic, octopi comes from the misconception that octopus is a second-declension noun in Latin when it’s actually a third-declension noun. If you were writing Latin, you’d still want octopodes