Henry Frederick Cope was a leading figure in the religious education movement in the early 1900s, and The Modern Sunday School is one of his attempts to drag Sunday school teaching into what he considered the modern age. He had strong opinions about how children learn, what should be taught at what ages, and how teachers should be trained.
The book was published in 1907, and that date is important. The educational psychology Cope draws on was new at the time. The structures he proposes feel obvious now because many of them were eventually adopted.
For anyone studying the history of American Protestantism, religious education, or progressive era reform movements, this is a primary source worth reading. As a practical guide for running a Sunday school today, it’s mostly of historical interest.