FLYING from Hongkong to the United States is simple enough. Reaching the main tourist destinations within the US is equally straightforward. But how about Dayton to Denver? Tampa to Tulsa? Little Rock to Big Pine? (and which one, California or Florida?) Increasingly, travellers are seeking the independence of go-where-you-please holidays and that is when the astounding complexity of internal American flights can become a real problem. The most convenient solution is the Official Airline Guides (OAG) Pocket Flight Guide, as carried in the briefcases of 300,000 of the world's most frequent flyers. Updated monthly, it carries details of more than 100,000 direct and connecting flights in North America, with other editions spanning the globe. OAG collects flight schedules direct from 730 different airlines and provides the database for all the 415 computerised reservation systems relied on by travel agents around the world.