There's a Justice League of America novel series out, and they're mostly very good. I still haven't gotten the Green Lantern book yet, but here's my reviews of the others.
JLA: Exterminators by Christopher Golden - Very good, actually my favorite of the series. Great characterization, and includes just about the whole DCU towards the end. Shows how the League has changed both in strategy and in maturity over the years. Highly recommended.
Batman: The Stone King by Alan Grant - Good, but not as good as the other books. The characterization is there, and I love how the focus on Bruce's fear is done, but the quasi-mystic nature of the storyline, as well as some annoying supporting characters turned me off. Still, I can see someone else loving this book, so I would recommend it to most people, especially Bat-fans.
Wonder Woman: Mythos by Carol Lay - Great book. Pretty much everything is excellently done here, except for some story problems that bothered me a bit. Highly recommended.
The Flash: Stop Motion by Mark Schultz - My second favorite book. Starts of a bit confusingly, but eventually this mystery begins to make sense, and reading it a second time, I picked up many clues I had missed the first time around. I recommend TWO readings, but one would be sufficient.
Superman: The Never-Ending Battle by Roger Stern - Very Good. The characterization is perfect, especially the ever-entertaining Bibbo Bibbowski, and the feats of heroism the JLA performs (especially the Flash) are quite impressive and well-written. The identity of the main villain was a mystery to me for a while, which led to more suspense, but more well-read DC comics fans might recognize him quicker. Not a must-read, but something Superman fans should pick up for sure.