THE MYSTERY AT REDTOP HILL is a Whitman Tween Age Book ("In-Between Books for In-Between Readers") by Marjory Schwalje and illustrations by Charles Geer. It's copyright 1965 by Whitman Publishing Company. I had no idea that the term "tween" had been around for more than fifty years!
Nancy, Tod and Steve are the protagonists in this mystery. I am sure it's good, but I have not read it.
I found this in a second-hand shop in Laconia, NH. Like I said, Redtop, schmedtop. I didn't care about the mystery, or the kids. I just fell in love with the illustrations. They were slapdash, confident and showed enormous life and skill.
Charles Geer (1922-2008), born in Long Island and educated at Dartmouth College, served in the US Navy during WWII. Afterward, he attended Pratt.
He illustrated many books, and enjoyed painting. He moved to Rockland, Maine, where he painted watercolors, and enjoyed sailing. He even built a couple of boats. Sounds like a guy I would like to hang out with!
Here are some of his illustrations from THE MYSTERY AT REDTOP HILL. This isn't quite half of them. And there are a good number of full page, gatefold illustrations. Lots of energy in these drawings and it made me a Charles Geer fan.