With his recommendation of the opera book, Brack reminds me that I did spend a lot of time reading music reference books, as opposed to music oriented narratives. The same can be said of movies. One of my faves was Trouser Press which is now entirely online. The focus here is on what was once known as alternative, which would now encompass death metal, indie rock, and a plethora of other subgenres. It's entirely content, so it is not as web-oriented as AllMusic, but they have strong opinions over there, so it is worth checking out. Rough Guides, the British travel and music guide maker, has some online content as well.