"Participant observation is the central research method of ethnography. It requires a researcher to engage with people in as many different situations as possible to look at what people actually do as well as what they say they do (as in interviews or documents) in their everyday lives" - from EAR Training Handbook
The following books are available as ebooks through the UCSF Library unless otherwise noted
Approaches to Ethnography by 
Participant Observation by 
Participant observation : a practical field guide for students and lecturers by 
Principles, Approaches and Issues in Participant Observation by This RSS feed contains the latest 5 articles for the Pubmed search: "Participant observation"