What are some good books and articles about the principal-agent problem? What are some good books and articles about the principal-agent problem? I'm looking for introductory academic books/articles or "popular-science" kind of ones.
In my PhD program everyone was using The Theory of Incentives: The Principal-Agent Model by Jean-Jacques Laffont and David Martimort. It offers a formal and relatively complete treatment without being too technical. I haven't read any other book on the topic, so unfortunately I cannot compare.

If you want to avoid any mathematical formalism, a good introductory article and literature review on the topic is Incentives in Principal-Agent Relationships by David E.M. Sappington, in the Journal of Economic Perspectives (1991).

Finally, I would also recommend that you have a look at the Nobel lectures by Hart and Holmström on contractual relationships inside organizations.