The following resources helped me to understand RRDtool better

rrdtutorial

Alex van den Bogaerdt's RRDtool tutorial

rrd-beginners

RRDtool Beginners' Guide

The man pages of rrdtool

man rrdtool, man rrdcreate, man rrdinfo, man rrdgraph, man rrdgraph_data, man rrdgraph_rpn, man rrdgraph_graph, man rrdgraph_examples
The online man pages can be found here.