This is a bound version of the manual that forms part of the GNU diff and patch package. The diff and patch tools provide you with a means of not only spotting differences between files but also distributing (source code) patches for your software. I have used diff a fair bit over the years and can vouch for it being a really useful tool.
Ten years ago, I might have considered buying this book; after all, it used to be time-consuming to load the manual pages into your favourite word processor and relatively expensive to run them off on your dot matrix printer, let alone binding the resulting document afterwards. Nowadays, I would not buy this book, given that I can download the manual in a selection of formats, including HTML, directly from the GNU web site. In fact, I have to admit to being rather baffled by the purpose of this book.
The back cover suggests that the publisher is donating $1 to GNU for each copy sold. Unless you particularly want a soft back, bound copy of the diff and patch manual, my advice would be the following: download the manual from the GNU web site, where you can be assured that it is completely up to date; then make a donation directly to GNU.