The following books and articles provide great research-based evidence for the importance of loose parts play (indoors and outdoors) and natural play spaces. They will likely lead you to other key resources, too!

Risky Play: Why Children Love It and Need It, by Peter Gray
Lenore Skenazy’s blog, Free‐Range Kids
A Playful Path, by Bernard Louis DeKoven
Managing Risk in Play Provision:Implementation guide, by David Ball, Tim Gill and Bernard Spiegal
Play and risk: Weighing up risks and benefits in children’s play, by Tim Gill for PlayWales
No Fear: Growing up in a risk averse society, by Tim Gill
The Playwork Primer, by Penny Wilson
The Theory of Loose Parts, by Simon Nicholson
Children without play, by Fraser Brown and Sophie Webb
Children’s Play in Natural Environments, by Children’s Play Information Service
Children’s right to play: An examination of the importance of play in the lives of children worldwide, By Stuart Lester and Wendy Russell (working papers in Early Childhood Development, No.57)
Play, naturally: A review of children’s natural play, by Stuart Lester and Martin Maudsley for PlayEngland