This "concise" edition of the significantly bulkier and heavier "New Penguin Opera Guide" (2001) includes no illustrations and covers "only" 250 operas (instead of the the other edition's roughly 2000 operas, not to mention 750 additional composers). Of course, the 250 works that ARE covered here include virtually all of the operas commonly performed. (But this edition does omit perhaps a handful of "borderline-popular" titles such as Belioz' "La damnation de Faust.")
Considering that this "concise" edition can be bought from Amazon's third-party sellers at a dirt-cheap price, it's a pretty good deal. But, whatever you do, don't pay the full retail price (of roughly $16). Heck, you could find a nice-condition copy of the unabridged "New Penguin Opera Guide" costing no more than that if you do some searching.
Of course, if neither "price" nor "compactness" is any concern to you, why even consider this edition? (Spend more and GET more!)
Finally, also be aware that perhaps the best (most comprehensive) single-volume "opera and composer encyclopedia" of them all is the original, 1993 version of "The New Penguin Opera Guide," which was titled "The Viking Opera Guide." That "Viking" edition (typically rather pricey in the used-book market!) has darker (easier-to-read) text and many more illustrations than its circa-2001 successor. Moreover, it covers somewhat more of the older composers and operas. [For more details, see my Amazon review (06-15-2007) of "The New Penguin Opera Guide."]