This book details the various ships and classes that comprised the world’s navies between 1906 (including a list of extant ships at the start of the period), and was the last of the first ‘tranche’ of releases (there was a follow-up 1947-1995 volume released in 1995) and it shows it, with a bit more detail on the extant ships at the start of the period (some of which is a valuable addition to the information presented in the 1860-1905 volume).
It's not the kind of book one would normally read cover-to-cover (and while I read it from start to finish, I did it in ‘stints’, rather than all at once, over a period of nearly two years) and is designed rather to dip in and out of, as a reference work. For this, both the overviews of each navy, and the information on ship classes is valuable.
The standard of research is excellent, but as would be expected in a huge, multi-author work of this scope, there are a few slips – these generally aren’t an issue, and it is likely someone interested enough in the ships of the period won’t be able to work out what’s going on.
All up, an almost indispensable reference work for people interested in the warships built between 1906 and 1921, and in my opinion the best of the Conway’s “All the World’s Fighting Ships” series (noting they’re all excellent). It’s a shame it hasn’t been reprinted (but it’s also understandable, as someone doing a reprint might want to integrate new information, and it would be a huge job).