The Knitter's Handy Guide to Yarn Requirements is more than a compilation of tables, listing yarn amounts by gauge.
For me, it means freedom from fear.
I used to say, loudly, at my knitting group, that I could probably come up with my own design ideas, but I had no idea how much yarn to use, and I hated running out of yarn (or having leftovers), so I had to rely on patterns. If for no other reason, patterns generally gave me a good idea of yarn amounts needed.
Now, I have more skills, more tools and more yarn, so I'm less afraid of those problems anyway, but it's not an exaggeration to say that I would never have knit my first "designed by me" sweater if it hadn't been for this booklet.
Not into sweaters? Yarn amounts are there for mittens, gloves, scarves, socks, hats, tams (need more than hats), and vets as well.
What more could you want for less than $6?