Drawing on the Right Side of the Brain by Betty Edwards
I got this book years ago and it was an immense help. The exercises in it will help you switch into the drawing mode of seeing and measuring with your eye. When you start to see things like the contour of a line, or negative white space, you start to understand the process of doing accurate drawings and how to reliably repeat that process.
You can buy it on Amazon. I see one in the "used" section for 14 cents. You can't afford not to have it for that price!