In my opinion, any fan of superheroes has to read The Wild Cards series. It is a set of books written, primarily, by a group of authors all editted by George R.R. Martin. This book series is a great example of everything GGE aspires to. It has the formulaic pieces of superhero fiction, but it is written for adults, no doubt about it. These stories are about "real" people. The characters are very deep and well characterized. The powers are not the focus of the story really. The powers are more the setting. It's a bit hard to explain, but it is so well done throughout the series. The first book came out in the 80's. I read it and was hooked immediately. The newest volume was released in December 2009. I have already devoured it and the series just continues to get better and better. A new book is coming out soon ("Busted Flush"). The books are basically written as "triads". Any one of the books is a good jumping on point. It makes it a richer universe if you start at the first and read all the way through, but all of them are just darn cool!
Now is a great time to get onboard.