In September 2009 I've been to Gokyo, Base Camp - Kalapattar and Chhukung, and since I was trekking alone, without guide or porter, I had the chance to check out most of the book. I found out that the directions are spot-on, and the maps almost dumb proof: the book is as good as a real guide, or even better.
The general info and Kathmandu chapters are good, but may be found on other guidebooks as well. The real highlights are the detailed walking routes and maps for the Khumbu area above Lukla. Everything was right there as detailed in the book, and following the indications I had a safe and easy trek.
If you go for independent trekking, either camping or lodge/teahouse, this book is really priceless: It's a great help planning your trek, and it's all what you need to go trekking on your own - in Kathmandu you just can get a bigger map of the Khumbu area and you're ready to go.
You may not need the book if you hire a guide: a good one will lead you on the right trail - which is impossible to miss, anyway - and tell you all what you need to know, but it is still helpful to know exactly where you are, and your options for an afternoon walk or a different viewpoint here and there, plus many other info and data about everything around there.
So, the bottom line is: find a place for this book in your backpack!