Google Books excerpt of Language, literacy, and learning in educational practice: a reader by Stierer and Maybin, see pp 45-58. This excerpt includes Vygotsky's work on proximal development. Also known as Vykotsky.
From the Learning Theories Knowledgebase at Learning-Theories.com, n.d. This page presents brief entries on Social Development Theory (Vygotsky) and Communities of Practice (Lave and Wenger), with links to further details for each.