This is without contest the easiest-reading of all Knuth works I'm aware of. Dealing with a pair in hiding from the greater society, a stone tablet describing a system of numbers is discovered and the duo learn of this system and make inferences to entertain themselves.
The book draws tasteful parallels between its creator character in the tablet and the Christian God. Being only a little ways over one hundred pages, with large fonts and in a dialogue style, this text is a nice story exemplifying mathematical thinking, even noting the advantages of this participatory approach. At a point in the work, a mistake is made, and later discovered and corrected, so readers would do good to stay alert.
This book is a pleasant and light read one should perhaps read before reading Knuth's greater works.