The cable stitches between the knit and the purl are often loose. What you can do to correct it is to purl that first stitch through the back loop. This will tighten it up. On the back, though, you have to remember to knit that particular stitch through the back loop as well to untwist it. It really makes a difference in the cable's appearance.
The Harmony Guides are excellent reference books--inexpensive, too.
Selvage stitches are the first and last stitches on something--it comes from 'self edge'--in patterns that use them.
P twist? - Is it done with two stitches, or is it a twisted purl, worked on 1 stitch?