Cool little puzzle with different level of difficulty. Some are really easy and other are challenging enough. They are mostly logico-mathematical problems. I like that the epub version as a link at the end of the puzzle that can bring you directly into the solution, so you don't have to go around into all the pages to find it and you don't find it at the bottom of the page, written head down like it's often the case. For those who like puzzle, it's a interesting book!