The Early Childhood Technical Assistance Center has the most comprehensive collection of research and resources for early childhood inclusion.
A few specific sources of research that we find most helpful are:
Brief Summary: Fact Sheet of Research on Preschool Inclusion (2014)
A list of 11 research points about preschool inclusion by Erin E. Barton & Barbara J. Smith
Inclusion for Preschool Children With Disabilities: What We Know and What We Should be Doing (2014)
A brief summary of key points we should be implementing based on the research regarding preschool inclusion by Phillip Strain.
Quality Indicators of Inclusive Early Childhood Programs/Practices: A Compilation of Selected Resources (2010)
A compilation of assessments and practices that support quality inclusion in early childhood classrooms.
Research Synthesis Points on Early Childhood Inclusion, NPDCI (2009)
A summary of key conclusions drawn from review of literature or research syntheses on early childhood inclusion.