Mixed, Hierarchical, and Multilevel Models are all the same thing, but Mixed Models generally refers to ANOVA models and Hierarchical and Multilevel Models generally refer to Regression Models.
The following are 3 very good references for learning about Multilevel Regression Models. They are in the order of increased technicality.
Arnold, Carolyn L. (1992) An Introduction to Hierarchical Linear Models. Measurement and Evaluation in Counseling and Development, 25, 58-90.
Plewis, Ian. (1998) Multilevel Models. Social Research Update, 23.
Singer, Judith D. (1998) Using SAS PROC MIXED to Fit Multilevel Models, Hierarchial Models, and Individual Growth Models. Journal of Educational and Behavioral Statistics, 24, 323-355.