The bold pictures are perfect for pointing out objects while reading; I remember using this to help my children learn to count. How many monkeys? How many caps? How many red caps? The text is simple but very rhythmic and fun to say out loud. The refrain of the monkeys, "Tsz, tsz, tsz", is one that children will have a lot of fun with.
This is the sort of book that I would buy two copies of if my children were very young-- one to read, and one to take apart and frame the pages! Of course, the best pages in the entire book are the two page spread of all of the monkey in the tree wearing hats, and that would be hard to frame!
There is a never before published sequel, More Caps for Sale, that has recently become available, and also a 1967 book, Circus Caps for Sale, that I will have to hunt down.