References to the different topics discussed in this contribution are given at the end of the relevant chapters. This is far from being a complete list, since it is impossible to review the hundreds of papers published each year on the subject. The emphasis is on papers discussing in detail some of the topics addressed in this contribution and papers that show typical examples of the observational material.
There are several comprehensive review articles on the subject that contain much of the material presented here. The more detailed ones are by Davidson and Netzer (1979), Osterbrock and Mathews (1986) and Lawrence (1987), as well as a few chapters in the second edition of the book by Osterbrock (1989). There is also a very complete book on the subject ("Astrophysics of Active Galaxies and Quasi-Stellar Objects", J.S. Miller editor, 1985) with many excellent reviews. The articles in that book most relevant to the present discussion are by Keel, Osterbrock, Ferland and Shields, Mathews and Capriotti and MacAlpine. Recent review papers on some of the topics can be found in the proceedings of the IAU Symposium no. 134 (1989).
For AGN classification see Osterbrock (1984, 1989), Lawrence (1987) and references therein.