This report by the Educational Testing Service provides an excellent discussion of a wide range of topics related to assessment for ELLs. The report begins by examining factors that influence ELL assessment, including students' language, educational background, and culture. There is also a discussion of the steps that should go into designing an appropriate assessment, as well as a description of testing accommodations that can help ensure that ELLs are treated equitably and that test results are valid.
Pitoniak, M.J., Young, J.W., Martiniello, M., King, T.C., Buteux, A., and Ginsburgh, M. (2009). Guidelines for the Assessment of English Language Learners. Princeton, New Jersey: Educational Testing Service.