In this recently published paper, Jianxin Wu helps the reader understand
how a CNN runs at the mathematical level. It is self contained and you should not need any further material to understand it from a mathematical viewpoint.
With CNN, the important part is understanding what happens when you adjust the different parameters. Bu in order to make sense of those it is much easier when you know the underlying principles behind it.
Here you go: https://cs.nju.edu.cn/wujx/paper/CNN.pdf