ATOM (A Taste of Mathematics) is a series of booklets published by The Canadian Mathematical Society (CMS). The series is designed for secondary school students and their teachers. The CMS is in the process of making all the volumes in this series available for free from their website at: https://cms.math.ca/publications/atom-booklets/
The latest volume, by Susan Milner, is on mathematical puzzles. Milner, along with two of the ATOM editors, wrote an article in the free CMS problem-solving journal Crux Mathematicorum about the topic of the first chapter of the upcoming volume of ATOM — the puzzle Rectangles. You can check out that article (and find the puzzles) here (Teaching Problems number16): https://cms.math.ca/publications/crux/issue/?volume=48&issue=4
A draft of chapter 2 from the upcoming ATOM, on the puzzle Three in a Row is available at: https://drive.google.com/file/d/1GKxuzgTQk_zbek1uZi_xipQME9mlxyHx/view?usp=sharing
Please visit the ATOM website listed above to learn more about the series and to download the full version of the booklet when it is available shortly.
Any questions about the ATOM series can be directed to the ATOM Editor-in-Chief, Shawn Godin, at godinmathstuff@gmail.com.