I made a last minute switch from straight needles to circular and a mistake didn’t get caught in the proofing process. Many of the “work into the back symbols” are incorrect.
There is an easy way to know which one to use. If the stitch above the “work in back” is a knit, then you knit into the back of the stitch. If the stitch above the “work in back” is a purl, then you purl into the back of the stitch.
I’ll contact the publisher and ask them to start an errata ASAP, then post the link here.