The 'At War' combined volume is the same as the individual volumes - but heavy! As with most of the 'At War' series, the main content is the photographs, with chapters covering development, specific events and anecdotal accounts from aircrews etc.
The P-47 book is the full development history of the type, along with chapters covering operational units of all air forces, accounts of actions, production listing, unit listings plus information on all 9th USAAF airfields in the UK and on the Continent. Some good photos, and representative colour profiles of 3 or 4 aircraft from each USAAF Air Force operator, plus allied use.
All in all, it appears to be a very useful reference, although I've only had a brief flip through it so far. Certainly well worth the low price for a 'as new' book.