This is far from a complete discussion of submarines of the world; much additional information is available elsewhere. However, it's a compact and competent discussion, and it includes much material that, although it can be found elsewhere, is scattered in so many different places as to take great effort to find. So I find it an exceedingly useful book.
I have two suggestions for a future edition. First, for the most interesting designs, such as the German Type 212/214, it would make the book far more useful to include a diagram of the sub's layout, instead of or in addition to photos. Few photos of modern subs reveal anything interesting, whereas diagrams of layout give an excelent idea of the advantages and disadvantages of the particular class. Such diagrams are available for the great majority of modern subs, although one must be careful to use the ones that are correct, not the ones created as disinformation.
Second, and even more important, a very brief bibliography on each important type of sub would make it ever so much easier to find the additional information any serious reader wants. Although every country has always tried to limit the information publicly available about its submarines, an enormous amount of infroation is available "out there". But, for example, locating the available information on the Russian Akula is a truly tedious exercise. The author has obviously found very many sources of such information, and with little effort could include in a future edition references to the sources he has located.
The author's omission of some of the most recent sub classes is understandable. If I consider the British Astute-class, for example, which is just briefly mentioned toward the end of the discussion of Trafalgar-class, Astute will not enter service until 2009 at the earliest, and although a surprising amount of informatioon about Astute can be found in various places, it seems to me inevitable that significant changes will be made between now and then, and that the need for yet further changes will be revealed by experience in sea trials and shakedown. Given that, and given the draconian nature of the British Official Secrets Act, which makes publication in Britain of even much of what is publicly available about Astute in other countries a criminal offense, the author's decision to say little about Astute seems to me a very reasnable diecision. (The book was created and published in Great Britain.)