A few come to mind, over the course of my childhood up to and including high-school.
The "Mad Scientist's Club" series
The Great Brain
Those "Encyclopedia Brown" stories
The "The Three Investigators" series
The Mysterious Island by Jules Verne
The original Doyle "Sherlock Holmes" canon
The Soul Of A New Machine by Tracy Kidder
The Lord of the Rings by Tolkien
Nineteen Eighty Four