Many great books have been written, and an increasing number are becoming part of the public domain. One of them, which I would like to point your attention to, is a classical book written by Benchara Branford in 1908! The title of the book is: “A Study of Mathematical Education, including The Teaching of Arithmetic”. Besides being an important book in the history of mathematics education, it also provides a nice insight into the teaching of mathematics as it was 100 years ago!
Personally, I think his very direct connection between the historical development of mathematics and the child’s development of mathematical thinking (often referred to as “the genetic approach” in mathematics education) is interesting.