I've also read theories - I don't know how scientifically robust they are - about sex selection via diet in an attempt to make the pH of the vagina more acid or alkaline. IIRC "girl" sperm do better in an acid pH (or more accurately, it kills off more of the weaker "boy" sperm), so some people recommend douching with diluted vinegar after sex.
The boy-sperm-are-weaker thing has also led to a lot of advice for the husband do things that are bad for sperm, like wearing tight briefs, taking hot baths etc. That mentality kind of makes me twitchy, as I wonder whether it could result in a damaged sperm fertilising the egg (leading, maybe, to miscarriage or birth defects or whatever). Possibly damaged sperm wouldn't be able to do the job, so it may not be an issue, but... I dunno. Makes me cringe to think of deliberately killing off sperm, somehow.
If you decide to go any of the natural(ish) routes, you should know it might take a while to conceive. Timing sex to 3-4 days before ovulation is HARD; and that plus all the other things I've mentioned could well lead to killing all the sperm in a cycle and/or reducing your DH's fertility, leading to no conception at all. Trying to conceive a girl is just more tricky than trying for a boy.
Then there are the high-tech methods, but they're expensive and, for a lot of people, morally problematic. That book certainly doesn't advocate "natural" anything, judging by the product description.