This page has the a collection of sources which we found useful, and which address play as a behaviour from a variety of different perspectives. There is quite a diverse range of things, and sorting them to make them more accessible is an on-going work-in-progress.
Articles and Books
Children's Play is More Than Physical Education, Avrid Bengtsson
Child in the City, Colin Ward [short extract]
Homo Ludens, Johan Huizinga
First Steps – A Primer on the Geographies of Childhood and Youth, ed John McKendrick
Dissolution of Children's Outdoor Play and its Consequences, Joe Frost
Child's Right to Play, eds Wendy Russel and Stewart Lester
The State of Urban Children, UNICEF
What Play Provision Should Do for Children, Playlink
Participation, Consultation and the Death of Play, Bob Hughes
Links
Paige Johnson's live archive of playgrounds arong the world play-scapes.com
Architektur fur Kinder's archive of playgrounds, books and historical images architekturfuerkinder.ch
Another world archive of play and playgrounds playgroundology.wordpress.com/
Tim Gill's fantastic and constantly relevant blog: rethinkingchildhood.com
Nil's Norman's research biblography on adventure playdismalgarden.com//research/ongoing-play-bibliography
Bernard Spiegal of PlayLinks also fantastic and evolving blog bernardspiegal.com