Looking for a book

I'm trying to find a book, but I don't have the slightest idea what it's
named.

The characters live in an enclosed valley. Sometime in the past, one of
their ancestors left in a hot air balloon to see the outside world. When he
came back, he fell out of the basket and hit his head, forgetting everything
he had seen. He had a picture of neatly trimmed trees, and a chart of a
family tree. The villagers couldn't figure out which was which, so they
assumed that the "family tree" in the notebook referred to the painting, so
they each planted a tree for their family and trimmed it neatly. The
"painting" of the family tree was hung in the museum, and all school
children from then on had to learn how to paint like that. He also had a
list of odd words like: Ave., inc., etc. He couldn't remember what they
meant, but they named all of their children these names, from that point on.

The main characters have to journey out of the valley, I believe because the
river had stopped flowing into the valley, and they had to find out why, but
I'm not positive. In the end, another race of people with large heads, I
believe they were called "mushrooms", tried to take over the valley. The
five or six main characters managed to gather all of the villagers together
to defend the valley.

Does this sound familiar to anyone?

Sounds like _The Gammage Cup_ by Carol Kendall, which was reprinted a
year or two ago.

--
"I like books... I mean some of them. I don't like the kind that gives
you the feeling they're trying to improve your mind. I like the kind that
tells you things you already almost know." -- _The Mummy Market_
A strange tale of _Cat and Mouse_: http:www.armory.com/~web/vol11.no4.html