Best introductory overview from the perspective of this class (simple, user friendly and uses SPSS):
Robert Bickel (2007) Multilevel Analysis for Applied Research: It's Just Regression! (Methodology in the Social Sciences). The Guilford Press.
Ita G.G. Kreft, Jan de Leeuw & Leona S. Aiken (1995) “The Effect of Different Forms of Centering in Hierarchical Linear Models.” Multivariate Behavioral Research 30(1): 1-21.
Enders, C.K. & Tofigh, D. (2007) “Centering predictor variables in cross-sectional multilevel models: A new look at an old issue.” Psychological Methods 12(2): 121-138.
Paccagnella, O. (2006). Centering or not centering in multilevel models? The role of the group mean and the assessment of group effects. Evaluation Review, 30(1), 66–85.
On Sample Size:
Stegmüller, Daniel. (2013). How Many Countries for Multilevel Modeling? A Comparison of Frequentist and Bayesian Approaches. American Journal of Political Science, 57(3), 748–761.
Rahim Moineddin, Flora I Matheson and Richard H Glazier (2007) “A simulation study of sample size for multilevel logistic regression models.” BMC Medical Research Methodology 7:34
Stephen W. Raudenbush and Anthony S. Bryk. (2002) Hierarchical Linear Models: Applications and Data Analysis Methods (second ed.). Sage
Joop Hox (2002, 2010) Multilevel Analysis: Techniques and Applications. Routledge
Tom A. B. Snijders and Roel Bosker (1999, 2011) Multilevel Analysis: An Introduction to Basic and Advanced Multilevel Modeling. Sage
For Longitudinal Multilevel Analysis:
Judith Singer and John Willett (2003) Applied Longitudinal Data Analysis: Modeling Change and Event Occurrence. Oxford University Press.
Raudenbush, S.W., Bryk, A.S., Cheong, Y.F., & Congdon, R.T. (2004). HLM 7: Hierarchical linear and nonlinear modeling. Scientific Software International. [Included with Demo and Full Version of the Software]
Rasbash, J., Steele, F., Browne, W.J. and Goldstein, H. (2009) A User’s Guide to MLwiN, v2.10. Centre for Multilevel Modelling, University of Bristol. http://www.bristol.ac.uk/cmm/software/mlwin/download/manual-print.pdf
Linda Muthen and Bengt Muthen (2010) Mplus. Statistic Analysis with Latent Variables. User’s Guide. Muthen and Muthen. http://www.statmodel.com/download/usersguide/Mplus%20Users%20Guide%20v6.pdf
Andrew Gelman and Jennifer Hill (2007) Data Analysis Using Regression and Multilevel/Hierarchical Models. Cambridge University Press.
Paul Bliese (2012) Multilevel Modeling in R. [though note that this is written by the author of the multilevel package in R and might have compatibility issues with other R packages like nlme or lme4]
Jose Pinheiro and Douglas Bates (2000, 2009) Mixed Effects Models in S and S-Plus. Springer.
Sophia Rabe-Hesketh and Anders Skrondal (2012) Multilevel and Longitudinal Modeling Using Stata, Volume I: Continuous Responses, Third Edition. Stata Press
Sophia Rabe-Hesketh and Anders Skrondal (2012) Multilevel and Longitudinal Modeling Using Stata, Volume II: Categorical Responses, Counts, and Survival, Third Edition. Stata Press
Heck, R.H., Thomas, S.L, and Tabata, L.N. (2010). Multilevel and longitudinal modelling with IBM SPSS. New York: Routledge.
Heck, R.H., Thomas, S.L, and Tabata, L.N. (2012). Multilevel Modeling of Categorical Outcomes Using IBM SPSS. New York: Routledge.