Textbooks
An Introduction to Active Galactic Nuclei
1997Peterson, B.
A gentle introduction to AGN at a level appropriate for our short course.Active Galactic Nuclei
2012Beckmann, V. and Shrader, C.
An X-ray astronomy centric view of AGN.Quasars and Active Galactic Nuclei: An Introduction
1999Kembhavi, A. and Narlikar, J.
A useful text for its clear exposition and good coverage of radio AGN.Active Galactic Nuclei: From the Central Black Hole to the Galactic Environment
1999Krolik, J.
A comprehensive and advanced treatment of most aspects of AGN physics.Active Galactic Nuclei (Saas-Fee Lectures)
1990Blandford, R., Netzer H., and Woltjer L.
Proceedings of a Saas-Fee lecture course by three pioneers of AGN physics. Dated in some aspects, but still useful for its treatment of the physical aspects.Fifty Years of Quasars
2012D'Onofrio, Marziano and Sulentic
A comprehensive conference proceedings with excellent articles on quasar research.
In addition to the specialised texts on AGN listed above, relevant chapters of introductory astronomy textbooks by Zeilek and Gregory, Schneider and others are also very useful to get a broad brush overview of the subject.
Review Articles
A fast moving field like AGNs benefits immensely from review articles summarising the various subfields. Here are some of the more recent review articles: