Odd eyes but all five fingers: Cherry Wilder, The Luck of Brin's Five (originally Angus & Robertson Publishers, 1979--I'm using the Harper Coll

The sequels are The Nearest Fire (1980) and TheTapestry Warriors.

In The Luck of Brin's Five , Dorn, child of a rural Five, finds a stranger in the lake. The Family adopt the stranger (Scott Gale) and absorb him into their society.

The book is very political. Most of the story is just about Gale and the Five as they run from a threat and as Gale finds out that these people are developing heavier than air flight. But there are hints at the ripples Gale causes in this early-modern society. Wilder, as might be expected, does an excellent job of depicting alienness, but her aliens are a bit too humanoid to be convincing (although that's necessary to her plot) and she makes the mistake of having Dorn continually tell us what words mean. We'd be better off clueless, or without the fake language in the first place as the explanations wreck the immersion.

At the end of the book Gale leads the family to find his party of scientists, and they are pursued by conservative elements. I felt dissatisfied because Wilder never really makes her readers work. All those ripples are left mostly unexplored. I was interested to see the default assumption that change is good, laid against also a default assumption that rural people are more authentic that city people. Sooner or later those two ideas will clash.

I started the sequel, The Nearest Fire but the early parts are just too similar: this time it's a female earther who is found. I gave up half way through. The books are good, but the sequel should probably be read on its own. I don't actually own the third in the series but will keep an eye out for it.