Mostly Harmless Econometrics: An Empiricist's Companion, Joshua D. Angrist and Jörn-Steffen Pischke, 2009 (Princeton University Press) - A classic and accessible textbook providing an intuitive introduction to causal inference methods, including a detailed treatment of Difference-in-Differences.
Econometric Analysis of Cross Section and Panel Data, Jeffrey M. Wooldridge, 2010 (MIT Press) - A comprehensive graduate-level econometrics textbook that provides a rigorous treatment of panel data methods, including Difference-in-Differences and fixed effects models.