\cite{goossens:companion} — [2]
\cite{goossens:companion,hagen:metafun} — [2, 1]
\cite[page 99ff]{goossens:companion} — [2, page 99ff]
\cite[page 99ff]{goossens:companion,hagen:metafun} — [2, 1, page 99ff]
Hans Hagen. Metafun. Preliminary Version October 27, 2000, October 2000. http://www.pragma-ade.com.
M. von Goossens, S. Rahtz, and F. Mittelbach. The LaTeX Graphics Companion. Addison-Wesley Publishing Company, Reading, Mass., 1997.