40 books
The Playground, Vol. 11: The World at Play; April, 1917 (Classic Reprint)
The Playground, Vol. 21: April, 1927 to March, 1928 (Classic Reprint)
The Playground, Vol. 4
Proceedings of the Second Annual Playground Congress
The Playground, Vol. 13
The Playground, Vol. 20: April, 1926; Year Book Number (Classic Reprint)
The Playground: April, 1929 (Classic Reprint)
The Playground, Vol. 15
The Playground, Vol. 15: April, 1921 (Large Text Classic Reprint)
The Playground, Vol. 17
Proceedings of the Third Annual Playground Congress, Pittsburg, Pa., May 11-14, 1909, and Year Book 1909 (Classic Reprint)
The Playground, Vol. 19: April, 1925; Year Book Number (Classic Reprint)
The Playground, Vol. 10
The Playground, Vol. 9
The Playground, Vol. 22
The Playground Volume 4
The Playground Volume 8
The Playground Volume 18
The Playground Volume 6
The Playground Volume 17
The Playground (Volume 11)
The Playground Volume 7
The Playground Volume 1-2
Proceedings of the Annual Playground Congress and Year Book Volume 3