Sources for crond, in each Linux distribution and at vix.com: ftp://ftp.vix.com/pub/vixie/
Debian Linux distribution: http://www.debian.org/
Debian cron web page: http://www.debian.org/Packages/stable/admin/cron.html
Debian anacron web page: http://www.debian.org/Packages/stable/admin/anacron.html
Red Hat Linux distribution: http://www.redhat.com/
Manual pages: cron(8) or crond(8), crontab(1), dh_installcron(1) (Debian-specific), run-parts(8), at(1)
Running Linux, 2nd Edition, Matt Welsh and Lar Kaufman, O'Reilly & Associates, Inc., 1996.
For other references, many Linux books provide useful examples of how to use cron.
All listings referred to in this article are available by anonymous download in the file ftp://ftp.ssc.com/pub/lj/listings/issue65/3290.tgz.