I've always thought it was amazing how two little stitches, the knit and purl could combine to make all sorts of things. Actually it's the combination (Knit 2 Together) and lack of (Yarn Over) these two stitches that creates numerious stitch patterns.
People have been creating different stitch patterns throughout the years and written books which contain many of these stitch patterns. Why would they go through the effort to figure out all of these combinations? One use for these guides is for anyone who wants to create a pattern to have some inspiration. Another use for these books is the fact that they give knitters semi-easy way to modify patterns to the stitches that they prefer. This may take a little effort and some math, but may take a pattern from ok to perfect for it's recepient.
In the past few years, Vogue knitting has released some very good stitch dictionaries. These books have very clear directions and good photographs of each of the stitches so that the knitter can get a good idea of what the finished pattern will look like. There are currently three books:
Vogue Knitting Stitchionary Volume One: Knit & Purl
The Vogue Knitting Stitchionary Volume Two: Cables
The Vogue Knitting Stitchionary Volume Three: Color Knitting
There are other stitch dictionaries out there, but these are my favorites. Their presentation uses very nice yarn in neutral yet beautiful tones, which makes it easy to imagine the pattern in any yarn.