As any kind of reference book on comics, this just doesn't cut it because of the limited number of titles mentioned which, because numerous titles have multiple entries, is considerably less than 1000. As a critical guide to comics it is, bluntly, a joke. 40 words per entry (actually this was the highest of several entries I counted) doesn't allow much space for anything other than the most superficial of comments. On these levels alone the book is a complete failure.
That doesn't mean, however, that it's a complete waste of time. What it does succeed at being is as a nice piece of nostalgia because of the wonderful and often garish covers it reprints. While it does cover a broad range of comics it's still difficult to find a page that doesn't have at least one super-hero title and usually more. This a colourful and undemanding fun and very lightweight read where the illustrations are far far more important than the text. If you're looking for a decent historical survey of American comics (manga barely gets a mention) then look elsewhere.
Update, 22nd Feb.
Actually, the more closely I look at this book, the more annoyed I get. Now you may well think I'm over-reacting about something which is basically designed to be a fun piece of trivia but it's inconsistent even on its own terms. The title alone, and this may well be its intent, is silly. Personally I'd call it unreasonable. Given that huge numbers of its entries are effectively unobtainable unless you're earning seven figures a year or have vast personal wealth, it's effectively impossible for you to even get your hands on hundreds of the titles included if they weren't published by Marvel or DC.
It's also unlikely that you'd enjoy them all even if you could. More than a few are pretty dire comics by most standards (several of them written by the author himself who is including them out of sheer mischief or misguided vanity -Astonishing Tales featuring It! The Living Colossus for example which I wouldn't include in a list of 50,000 titles) and included for historical reasons or appalling taste on the part of the writer. That apart, because of the sheer variety included no-one will have a taste broad enough to enjoy them all. I doubt if there are many, if any, British readers who understand the appeal or, and actually enjoy, the Archie-Betty-Jughead-Veronica long-running series.
Isabella is also inconsistent in that he includes Graphic Novels and Trade Editions (i.e. collected series). I don't mind him including these but he's inconsistent even on his own terms by including the comic Love & Rockets by the Hernandez Brothers but also the massive hardback collections Palomar and Locas which originally appeared there. I said earlier that Manga doesn't get much of a look in though he does include the American edition of the graphic story by Osamu Tezuka of MW (a brilliant piece of work) but omits Tezuka's seminal and widely available Astro Boy series.
I could go on but I've spent enough time on what is basically an otherwise likeable piece of nostalgic ephemera.