If you like beer history, you'll like this book. About half of it is history of the IPA from 1800 to present. The history is well written and extremely well researched. If you are the sort who reads the beer history blog Shut Up about Barclay Perkins, you will enjoy the book.
Unfortunately, I found the book lacking in real tips for brewing better (modern) IPAs. There is good recipe information for brewing historical IPAs circa 1800-1850 (including water chemistry), but none of the recipes are scaled for the standard 5 gallon homebrewer setup. I myself am looking forward to using the information to brew a heavily hopped Burton IPA in the near future. There are also excellent recipes for some of the best-known modern IPAs in the country. All in all, about 50 pages of recipes. These recipes are from the horse's mouth, so to speak, and should be much more accurate than anything you find on the internet.
There is some style discussion for double IPAs, black IPAs (or dark cascadians, whatever you want to call them), and Belgian IPAs, but it really doesn't amount to much and is more of a side note than a main focus.
There is good brewing information in the book, but if you have already read Palmer's How to Brew and Yeast by White and Zainesheff (both of which are excellent books all homebrewers should read), I don't think there will be anything new to you. In the end, it is a nice addition to the interested brewer, but definitely not a must-have. Cheers.