I have just finished reading a wounderfull and enlightening karate book called, Hidden Karate: the true bunkai for the Pinan / Heian katas and Naihanchi. The author is a gentleman called Gennosuke Higaki.
I have a select collection of karate books by past masters, this book however is one of the finest karate manuals i have ever read, it has a wealth of information and some really great bunkai.
In this great book Gennousuki Higaki is brutally honest, answers many questions regarding unusable katas, goes in depth about the great Master Itosu and the Pinan Katas. In one section he clearly states that Master Itosu taught the block and punch combinations to children, and taught his adult students at his home.
It should also be known that were different variations of the Pinan / Heian katas at the time of Itosu, why was this, because it was Master Itosu who taught those variations in the first place and thats why we have this today.
In this book it claims Itosu developed these five forms to educate instructors and if one trained 2 to 3 hours a day, over 3 to 4 years one would possibly be able to understand the hidden parts of the katas. It also explains the mystery behind the dangerous moves being removed from the Pinan / Heian katas, this as many of us know this is not the case. Those dangerous moves are indeed still there, however depending how you approach those katas, for example if you believe that these katas are for children or only beginner katas then sadly you will never understand these great katas or the spirit behind Master Itosu who created them.
I highly recommend this fine karate manual and deserves to be in any serious karate-ka's library or martial artist in general, it can be found on amazon or ebay etc.
Hope this is of some interest.