"The arguments for girls-only education remain as strong as ever: the academic excellence of the top schools and the benefits of being educated in a place where your role models are women," the guide says.
"What seems to have faded are the voices loudly extolling these virtues.
Girls' schools are not blowing their trumpets as loudly or as confidently as once they did."
Some schools have suffered a crisis of confidence over not having the resources to offer top facilities and a wide range of subjects, according to the guide.
"Many are moving to co-ed sixth forms of doubtful attractiveness or even offering inducements. We have heard of palm-tops being pressed into the hands of wavering fifth-formers to get them to stay," it says.
The guide now covers more alternative schools, such as Steiner schools. It includes an entry for Summerhill, in Suffolk, where pupils attend lessons only if they want to.
In contrast, increasing pressure on young children trying to win places at top private schools is also highlighted.
"The continuing push to the academic in independent schools means some London schools are expecting three-and-a-half-year-olds to recognise numbers and words. Before long they'll have to compose a haiku too," said the guide's publisher, Ralph Lucas.