I don't think I've read a modern DIY book that was so blunt: fitting problems may also caused by "flat buttocks", a "pigeon chest" or "dowager's hump." I was genuinely surprised (and disappointed) that "fat ass" was not included.
First, and above all, a low bustline will detract from what otherwise could be a youthful appearance. For this reason, if you have an inkling that a low bustline is the cause of your fitting problems, adjust the straps of your foundation garments before you adjust your patterns or fit your garments.
Incidentally, The Vogue Sewing Book is great. An invaluable companion to Built By Wendy Dresses, which gives only brief information on sewing and adjusting patterns. I've seen it in thrift stores many times over the years, and it's definitely worth picking up.