One of the great things about a PDA (I have a Palm Tungsten T) is that you can store and carry around dozens of useful and interesting books to peruse whenever you have the odd five minutes between meetings.
For me one of these texts is Sun Tzu's The Art of War. Despite being written more than 3,000 years ago it still contains many useful insights into conducting business, public relations or politics in the modern world.
At some point in the near future I'm going to try and create a NetworxPR branded PDF copy of it for people to download for free.