I've been a subscriber to Decibel Magazine since its 4th issue - between that and the UK's Terrorizer I'm able to get a pretty comprehensive look at metal from a global perspective. The crowning feature in Decibel is their Hall of Fame feature - which takes an in-depth look at a "classic" metal album by interviewing everyone who was involved in its creation. The caveat for choosing an album for induction is that all members must be alive and willing to talk, so while there are some surprising exclusions from the HOF (no early Metallica, no Death), it actually makes for some great, interesting choices, and continues to be a recommendation for me if they record in question is one I don't already have.
Precious Metal: Decibel Presents the Stories Behind 25 Extreme Metal Masterpieces is an expanded look of 25 of the 50+ Hall of Fame inductees the magazine has reviewed so far. Edited by Albert Mudrian, who penned the excellent Choosing Death: A History of Death Metal (he's also the head guy at Decibel), there's not a lot here that I haven't already read, but in many instances the entries have been expanded and updated past the word count limits imposed by the magazine. You get a bew entry on Darkthrone's Transylvanian Hunger (they were "officially" represented in the magazine by A Blaze in the Northern Sky) and classic entries like Black Sabbath's Heaven & Hell (a nice, surprising choice), Slayer's Reign in Hell, At the Gates' Slaughter of the Soul, Napalm Death's Scum, and more by Cannibal Corpse, Morbid Angel, and Repulsion. But you also get less-known but incredible offerings by Opeth, Converge, Botch, and great examples, embracing black, death, hardcore, grind, and other subsets of the genre.
If you love extreme metal, you owe it to yourself to pick up Precious Metal or, better yet, subscribe to the magazine. But even casual fans can get a lot out of the book - if nothing else it makes an essential shopping list and a great "where to begin" for those looking to get deeper into a genre of music that's often ridiculed by those outside of the "know".