I have just returned from an 8-day walk along the 100 miles of the South Downs Way and found this guide invaluable. We walked the Winchester-Eastbourne route, which is the direction that the book is set out, but the maps and information could easily be read in reverse. The information about places to stay and eat is excellent and the maps are very detailed and easy to follow. You could probably get away with just taking this book and no Ordanance Survey maps as the maps in the book are so good and the Way is well marked, however it is always a good idea to have a back-up OS sheet in case you do wander off the track (it happened to us once or twice when we were really tired and not paying as much attention to the trail).
We camped along the way and the advice about distances and suggested itineraries were really good and made the overall distance manageable. The book is a nice size to carry and fits in rucksack pockets to be easily accessible. There are lots of other interesting features noted on the maps and several boxes of interesting facts about the area through which you are walking.
The only minor criticism was that the occasional gradient was marked the wrong way round - a bit of a disappointment when you've just trudged up several long hills and the map promises a section of downhill... only to find the arrow was marked the wrong way and it's actually another uphill! Otherwise a great little book and highly recommended, if not essential, to those planning on walking some or all of the South Downs Way.