The book is something of a standard reference amongst watch collectors and there are many reasons for it. It has lots of information on diverse aspects that would educate the less than expert watch collector (movement details, designs, etc.) on top of the main content, which is being a watch price guide.
The main section is roughly divided between pocket and wristwatches and each watch covered gets a small BW picture and the expected price (depending on condition). The prices are in US$ and probably more representative of values in the US - although a case could be made that these are closely representative of European prices (watch collecting having gotten increasingly international).
The pictures are small and not particularly sharp so do not expect to be able to see the finer differences between similar versions of a watch and in order to make the book at a price acceptable to the average buyer, you cannot expect coffee table book quality of photos or production (especially not at 1200+ pages).
Some have also commented that the US focus means lots of US watches being covered, with a relative dearth of non-Swiss European ones. This is so, however a possibly larger problem is that the coverage stops somewhere in the 1970s / 1980s, meaning than many more current but very much collectible pieces are not included.
Still, if your focus is on pocket watches or more vintage wristwatches, the guide is very good and inspite of the mentioned criticisms well worth buying for the average watch collector. Whether it is worthwhile investing into each annual addition will be primarily dependent on your individual passion / collecting but a case can definitely be made for getting one every couple of years.