Roger A. Wojtkiewicz 
Elementary Regression Modeling [PDF ebook] 
A Discrete Approach

Elementary Regression Modeling builds on simple differences between groups to explain regression and regression modeling. User-friendly and immediately accessible, this book gives readers a thorough understanding of control modeling, interaction modeling, modeling linearity with spline variables, and creating research hypotheses that serve as a conceptual basis for many of the processes and procedures quantitative researchers follow when conducting regression analyses.