Dishcloths on the Double 8 Exciting New Designs for You to Crochet Using Your Double-Ended Hook and Worsted-Weight Yarn (Crochet on the Double): McClain, Jennifer Christiansen (designs), Illustrated: Amazon.com: Books
Math for Knitters: Knitting with Yarn Held Double
What is Worsted Weight Yarn? - A Beginner's Guide - Sheep and Stitch