One such "single concept" book is Play. In this story, each page has one simple sentence about how children play around the world. For beginning readers it's awesome because the sentences are really simple yet not generic or silly (like I find so many easy readers to be). Big Boy loves looking at the photographs of the different children in this book and commenting on what they are doing. The pages are great vocabulary builders for preschoolers. In addition, there is a section at the back of the book that identifies the country in which each photo was taken and includes a brief explanation of the activity portrayed.
If you are looking for some quality and simple nonfiction for your child, Ann Morris is a great place to start! ENJOY :)