Is there a good reference for learning about flag varieties $G/P$? I'm already comfortable with the algebraic geometry and the example of Grassmannians, but I am not so comfortable with algebraic groups aside from basic Lie theory.
For example, it would be nice to know which properties of the Grassmannian extend to more general $G/P$ (decomposition into Schubert cells, rules for intersecting classes, the Picard group and the Plücker embedding), so I know what sorts of arguments generalize cheaply and what arguments do not.
