I really like that book a lot.
I doubled the yarn in the heel area, but it wasn't a really good idea. It's a little too heavy for normal shoes, but since I wear backless shoes a lot, it's okay.
Regular single strand would have been fine.
Where I really need replaceable areas are at the toes - they always wear out there first.
This is a mix of blue, purple, teal, with heels & toes dark grey. Wool/nylon mix. 2008.