The original article by Luke Mewburn offers a general overview of rc.d and detailed rationale for its design decisions. It provides insight on the whole rc.d framework and its place in a modern BSD operating system.
The manual pages rc(8), rc.subr(8) , and rcorder(8) document the rc.d components in great detail. You cannot fully use the rc.d power without studying the manual pages and referring to them while writing your own scripts.
The major source of working, real-life examples is /etc/rc.d in a live system. Its contents are easy and pleasant to read because most rough corners are hidden deep in rc.subr(8). Keep in mind though that the /etc/rc.d scripts were not written by angels, so they might suffer from bugs and suboptimal design decisions. Now you can improve them!