5 thunders is not taught lightly, and most who teach it do not believe just anyone can learn it. Thus some have taught it, and i have seen parts of it in books.
the mystery of the 5 thunders (wu xing chen) is that is it a secret word,.. written with the character for Thunder five times.
Fotr example, if you write the character Thunder three times, it means "The roar of the Thunder"
Thus it is not "about Five thunders",.. but rather it is about something that has to do with thunder.
I am not supposed to tell you what it means, but I can provide a hint.
It could be said to mean "hitting with the hand that claps and makes thunder."
the wu xing chen is one of the mysteries of the power of Chi sacred to the fire school of Taoism, specifically those who have had their names burned up to the dragon (those who have sacrificed their base identity to become vessels of the immortal Self the ego clouds).
and now for the legend:
One who is the victim of a full attack of the five thunders is said to be hit with all the power of the bloodline of the emperors of the Yellow Kingdom. it is for that reason that if you attack someone with it full force, it will kill you as well, unless you be of royal blood, or so it is said. (and yes this is a metaphor)
as far as teaching it or learning it from a book, the 64 secret songs are publicly sang and rarely understood. translations of them do little to reinforce the Chi, for most of the meaning is lost.
in huk kung, it is not "Iron Fist",.. rather the word for "Iron" and the word for "fist" form the word zhang jin, which roughy means "Invincible Palm which can hold great fire and not be made formless" (i.e. not melt)"
The comic book came about when martial arts became popular, and no one here knew chinese. they just took the term for "Qi kung jin zhang" (Spirit Fist Iron Palm) and made the decision to make "Iron fist" his name,.. and fit well into the western ideas of power.. and then they took liberally from the stories of parcival and The magic flute to make a stroy of initiation after personal mishap that imbued a protagonist with a great power and a ethnic sidekick who would be his superior, were it not for this great gift. (my opinion, from studying western opera and eastern martial arts)
look up iron palm and you can find some stuff, be discriminating. it has become a buzz word.
here is a overview of my study, of which "Iron Palm" is a part of the exercise and discipline
http://home.comcast.net/~baguazhang/bagua.html
(but this is not from my xi fu, nor my school, who do not have a website. Jian Chang from Taiwan, who flew here to meet me, if you are curious.)