This book is not entirely accurate though.
First off clitoris is not just limited to the head of the vagina. It trenches much deeper.
Scientists claim there's no such thing as penetration orgasm as it's merely another form of clitoris orgasm.
That's the primary reason why girth is also an important factor.
Second, Porchio(=cervix orgasm) is a very controversial topic.
Some(more like few) believes it gives you the best orgasm possible if you develop this spot, while MANY women believe that even the idea of their cervix getting hit cause them pain like when men are kicked in the balls.
Scientists also say there's not enough nerve clusters located at cervix to get an orgasm.
However there are people who can get orgasm with nipple stimulation alone so who knows.
But if you ask me I think many people confuse cervix orgasm with A-spot, an erogenous zone known to be located "behind"(towards anus) cervix.

Base +6, rtypeend -9, boolp +13