Deathwatch by Robb White is $1.99 in the US right now. Per its blurb and Wikipedia, it won the 1973 Edgar Award for Best Juvenile Mystery, although reading the blurb, it doesn’t sound all that much like a juvenile mystery to me. I do have a vague memory of reading this when I was really young, though, so maybe that’s how it was marketed. Deathwatch has also been made into a movie a couple of times. And my thanks to @ZodWallop on the MobileRead forums, whose post alerted me to the sale.