I remember these short 20-30 page math books written for 6th-8th grade students by prominent Soviet mathematicians. Extremely clear exposition. About 30 years later, I still remember the contents of the book on logarithms, which gave geometric interpretation of log as the area under the curve of 1/x, and used that to show why log must satisfy properties such as log(ab) = log(a) + log(b). As a 12 year old, my mind was blown away and I read all such books that I could find, which lead to a lifelong passion for mathematics.