This pair of full-colour resource books for Early Years practitioners are feely available online from Northern Ireland Curriculum.
1. Learning Outdoors in the Early Years
This 158-pages book was first published in 2005, before Forest Schools really took off. It is a useful resource for learning outdoors immediately outside your setting, rather than in a remote woodland. It is a 3MB download.
2. Learning Through Play in the Early Years 
This 163-pages resource book was compiled by the Early Years Interboard panel in response to requests for guidelines on provision and progression in play. It is a 5.2MB download.
Both books are very attractive, with lots of charts and bullet-point lists. They are not textbooks; instead, with their full-colour photographs, they really stimulate your thinking – I became increasingly impatient to try out some of their ideas! I can imagine pages from these books working really well as slides projected onto a wall or screen during staff or volunteer training sessions – just select the pages that correspond with the topics you are covering.
For anyone interested in the academic provenance the above two books, Northern Ireland Curriculum also hosts a collection of research on the Early Years Enriched Curriculum by Queen’s University Belfast, including a very useful literature review of play-based pedagogy.
Exploring Play: The Role of Play in Everyday Life
cyp-media.org 