These can give you an overview of a subject, help you focus your topic idea, and tell you what the classic studies in the field are.
Encyclopedia of Infant and Early Childhood Development. Marshall M. Haith and Janette B. Benson, eds. Academic Press, 2008. 3 vols. Available online.
The Cambridge Encyclopedia of Child Development. Brian Hopkins, ed. Cambridge University Press, 2005. Available online.
Early Childhood Qualitative Research. J. Amos Hatch, ed. Routledge, 2007. LB1139.23 .E2743 2007