Darling clearly had the non-technical reader in mind when he wrote up his descriptions, as he steers well clear of jargon (in a jargon-laden industry), and I appreciate that he kept some descriptions very short. For spaceflight terms the book functions as a dictionary, and the explanations are kept to a few sentences. For other topics, the book functions more like an encyclopedia; in some cases several pages are dedicated to a single topic (Gemini Program, spacesuits, etc).
If Darling were standing in front of me, and asked me... "well, what do you think? Is it complete?" I'd have to say yes. It's complete. Everything that has anything to do with spaceflight is in there. I've found it useful to consult entries before writing up some of my own stories; especially if it's been several years since I last wrote about a subject (although some space agencies have great press material, many of the aerospace firms provide descriptions of their own programs drenched in marketing-speak).
Taking its cue from its encyclopedic parent, The Complete Book of Spaceflight is liberally sprinkled with photographs, sidebars and tables of information. Unfortunately, the pages are all black-and-white, so you don't get to see any of the images in colour. I wish the publisher could have splurged on full-colour printing - this would let the book spend equal time on your desk and coffee table (maybe they'll consider it for a future edition?).
The other problem, and this is no fault of the author, is that the business of space exploration is still unfolding. Events in the last few months would have already rewritten chunks of the book (Columbia, Rosetta), so it would be cool to see some kind of Internet site with updates.
I think you'd be happy to have The Complete Book of Spaceflight sitting on your desk or in your bookshelf, standing by to help you navigate some of the more obscure space news journals.