In addition to the primary literature, I have drawn on a number of reviews, books, and personal communications. For the early work in radio astronomy, the books by Sullivan (1982, 1984) were informative and enjoyable; the former conveniently reproduces many of the classic papers. The book by Burbidge and Burbidge (1967) was an invaluable guide. A brief summary of early studies is contained in the introduction to Osterbrock's (1989) book. The Conference on Seyfert Galaxies and Related Objects (Pacholczyk and Weymann 1968) makes fascinating reading today. The status of AGN research in the late 1970s is indicated by the Pittsburgh Conference on BL Lac Objects (Wolfe 1978). Many aspects of AGN are discussed in the volume in honor of Professor Donald E. Osterbrock (Miller 1985), which remains of interest both from an historical and a modern perspective.
Review articles that especially influenced this work include those by Bregman (1990) on the continuum; Mushotzky, Done, and Pounds (1993) and Bradt, Ohashi, and Pounds (1992) on X-rays; and Stein and Soifer (1983) on dust in galaxies. Historical details of the discovery of QSO redshifts are given by Schmidt (1983, 1990); and an historical account of early AGN studies is given in the introduction to the volume by Robinson et al. (1964). A comprehensive early review of AGN was given by Burbidge (1967b). A review of superluminal radio sources is given by Kellermann (1985), and the emission-line regions are reviewed by Osterbrock and Mathews (1986). A succinct review of important papers in the history of AGN research is given by Trimble (1992).
Recent books on AGN include those of Krolik (1999), Peterson (1997), and Robson (1996). Many interesting articles are contained in the volume edited by Arav et al. (1997). Recent technical reviews include those by Koratkar and Blaes (1999) on the disk continuum; Antonucci (1993) and Urry and Padovani (1995) on unified models; Lauroesch et al. (1996) on absorption lines and chemical evolution; Ulrich, Maraschi, and Urry (1997) on variability; and Hewett and Foltz (1994) on quasar surveys.
The author is indebted to many colleagues for valuable communications and comments on the manuscript, including Stu Bowyer, Geoff and Margaret Burbidge, Marshall Cohen, Suzy Collin, Martin Elvis, Jesse Greenstein, Ken Kellermann, Matt Malkan, Bill Mathews, Richard Mushotzky, Gerry Neugebauer, Bev Oke, Martin Rees, George Rieke, Maarten Schmidt, Woody Sullivan, Marie-Helene Ulrich, and Bev and Derek Wills. Don Osterbrock was especially supportive and helpful. This article was written in part during visits to the Department of Space Physics and Astronomy, Rice University; Lick Observatory; and the Institute for Theoretical Physics, University of California, Santa Barbara. The hospitality of these institutions is gratefully acknowledged. This work was supported in part by The Texas Higher Education Coordinating Board.