This book, one of the seemingly-endless successions of Dummies, For Idiots, Everything, Introductory books, is an interesting mix of What Makes Comics Cool coupled with everything goofy about our beloved medium.
For such a small package, The Rough Guide to Superheroes manages to cover a lot of ground. Of course, it's all in small, easily-digestible chunks, but that's exactly why it works. Not for you, of course; you already know who the one, true Green Lantern is, and moreover, you know exactly WHY the movie Batman & Robin was such a colossal failure. But your supportive but clueless girlfriend, or the guy in the cubicle next to you who seems fascinated by your action figures but doesn't quite know what they all are, will find it helpful.
That's where the Rough Guide shines: It's familiar, with lots of Benday Dots and splashy TV Batman-style "booms" and "cracks" to punctuate it's hundreds of different entries on the major superheroes, the comics history, the creators, and the pop cultural references.
But there's nothing really earth-shattering about the book, either. For the already indoctrinated, you won't find much of interest except maybe the chapter on context, wherein the contributors touch on WHY comics are what they are to so many different people (and yes, the phrase "post-9/11" is used several times). It's a shame that the 26-page chapter comes so late in the book. You may want your current girlfriend to start there instead of at the front, where you'll find simplified bios of Alan Moore, Bob Kane, and other people she's never heard of.
The Rough Guide to Superheroes is a nice introduction to the oeuvre, even though it throws up some of the absurdities of the genre and includes some real head-scratchers, such as Super Mario. Inconsistencies aside, and humorous asides aside, I'd recommend it to the novice only, or the die-hard completist (as if such a thing ever existed in the world of comics).