The excellent introduction to Victor Sogen Hori's book Zen Sand had been made available online as a free download (pdf) by the Nanzan Institute for Religion and Culture.
Zen Sand is Hori's translation of a collection of jakugo (capping phrases) used in Rinzai training. Hori's introduction, however, contains a wealth of general information on koan practice. Chapter 2 of this is particularly useful to anyone wanting to understand how koan are used in Rinzai Zen: "In this chapter, we will consider the stages involved in kõan practice as well as some of the technical terminology that accompanies it. The aim is to present a general picture of the overall training career of full-time practitioners engaged in the kõan curriculum."
http://nirc.nanzan-u.ac.jp/publications ... ion%29.pdf