This book does an excellent job of including biographies and listed discographies of many representative bands in metal separated by categories such as style of music (like Power Metal), geography (like South American Metal), combinations of these (like UK Trash) and a few others (like 70s Metal and Innovative Bands). So you will see a collection of bands from Black Sabbath to Lordi, from Metallica to Cirith Ungol, from Japanese Ezo to Italian Rhapsody (of Fire), from legends to obscure road warriors.
What you won't be seeing, though, are some controversial genres. I'm talking about Hair Metal, Grunge, Nu Metal and bands that the author perhaps thought were split between Hard Rock and Metal (the aforementioned 70s Metal category is rather short, but the bands there are very crucial to Metal's history). So there is no Motley Crue, no Nirvana, no Korn and no Deep Purple, for example. This is not necessarily a bad thing, since a book focusing in band biographies could be three times the size of this one and still leave out interesting bands. So, all the bands in this book are inarguably metal and reading this book can get you to know what kind of bands are (or were) out there.
However, the thing I expected a little more from this book was a general focus on the music itself, not only the story of the band. Yes, you will know what kind of sound is the trademark of every band (usually at the beginning of the entry) and if a band changed the face of metal in big or subtle ways that comes across in the text as well. But the entries can't have the same length (bands like Infernal Majesty never had the long career of Black Sabbath) and often I want to read more about which albums by Trouble are the classics, not what happened to them between albums or which bands they toured with.
Bottom line, this book is a great read (some cool photos as well) but if you want to expand your knowledge of the bands AND their music, pair this book with the ones by Martin Popoff's, most of which have excellent reviews of whole discographies, and even of individual songs.