Disclaimer: I do not know about this book in particular because I
haven't read it, but I am familiar with pranayama, so please take my
words from this perspective.

"p. blauw"  wrote:
>Why should pranayama NOT be exercised from a book?
>Apart from hyperventilating (breathing too fast, too much oxygen) what can
>go wrong?

You can disrupt your pranic cycles and damage chakras and nadis, which
can in mild cases lead to disorders in functioning of the heart and
lungs and other organs (I've heard of one person who has damaged his
heart with invalid practice of breath retention), and in more serious
cases you can go insane and/or die. Of course, that doesn't apply to
all pranayamas - for instance, bhastrika is mostly harmless, but if
you attempt to force some fixed inhalation-retention-exhalation ratio,
you might experience serious problems.

>What is it that should be approached with care and under a teacher's
>supervision?

If you practice a useless technique, which doesn't produce any results
whatsoever, then there is no danger. But if something is effective,
then it works like any medicine - dangerous if misused. Some
pranayamas are probably the most effective means of influencing the
energetic system. Some are merely imitation of some spontaneous action
(those are the most dangerous ones). Some are effective only on the
physical level of breathing, and therefore spiritually useless. Some
(such as for instance breathing through susumna) can kick your astral
into Pluto's orbit in less than a minute (in a very negative meaning
of the metaphor), if you're mad enough to try them and your mind isn't
perfectly still.
It's not enough for the breathing itself to be performed correctly,
one must have the right focus of the mind, the body has to be in a
right state, and only then it can work. For instance, kumbhaka (breath
retention) takes place automatically when one's spirit observes the
inner Divine states. If you observe a person in such a state, you'll
see that his/her eyes are pointed upwards, that the inhalation is
slowed down and prolonged almost infinitely, as that person inhales
the bliss of the inner vision, and that the tongue is touching the
palate, spontaneously going backwards (kecari mudra). If one attempts
to imitate this, one can either achieve nothing or damage one's
system, because such imitation won't lead to the Divine state, it's
merely a side effect; living in Monte Carlo can be a side-effect of
being rich, but if you're poor and you go to Monte Carlo in order to
become rich, you'll most likely starve. The same applies to asanas,
mudras and other techniques. One has to make a distinction between the
means and the goal, because the symptoms of the goal are often
perscribed as "techniques" of achieving the goal.

-- 
Web (Kundalini-yoga): http://danijel.cjb.net