SAS Survival App

Reviewing information about wilderness camping I came upon the best selling SAS Survival Guide by John Wiseman.  It was interesting that they are pushing the associated IPhone App on the cover of the book.  Two versions exist, a free Lite version and a $5 full version.  I downloaded the free version and scanned it.  Remarkably good example of delivering knowledge as an App.  It contains lots of useful information, not all that is in the 380 page book. Very simply presented, as an index and documents.  But should I depend on a powered device in the woods?    An example of  how information is now shared between text and mobile devices.  The future?