I recently checked out the book Teach Yourself Visually Knitting Design by Sharon Turner. It is a great book both for the new knitter and the experienced knitter. I fall somewhere in the middle and have so much more to learn about this wonderful craft. I would definitely recommend this book to better understand the mathematics involved in designing knits. The first chapter especially has helped me understand gauge ranges. On page 7 there is a Standard Yarn Weight System Chart which lists the different yarn weights by category name with the recommended needle size to use. Let's say you prefer to knit with Medium Worsted yarn. The right needle sizes for this yarn weight in U.S. size range is 7 - 9.
Have you ever looked at a pattern in a book which requires a specific yarn but does not give information about the yarn weight (whether the yarn is sport or worsted weight?) The master patterns provided for shawls, hats, bags, sweaters, socks, etc... give detailed information about the approximated yardage needed, how many stitches to cast on based on the gauge of each different yarn weight. Knitting books stress the importance of making a gauge swatch, this one does too. Do take the time to make one to insure that your finished product will fit.
One of our library's borrowers shared with me the name of this website: http://wiseneedle.com
This website provides helpful information about yarn by manufacturer's name and fiber content.