I personally get a lot out of the Martin Popov books. I can read his reviews and pretty much tell that I'm going to like something even if it's something he pans. The downside for metal guys though is that he (like me) is going to waste a lot of space writing about things tangential to metal such as vaguely hard rock, etc. He is scheduled to have a 90's encyclopedia coming out soon, and that one should be pretty much pure metal reviews though.
There was also an Encycopedia of "Extreme Metal" that came out a couple of years ago that is pretty good, sorry I forgot the author's name. The M.C. Strong discographies are also very helpful books.