I know these books have been out for some time now but I only recently acquired the second one and I wanted to spend a little time talking about just how great they are.
I think of myself as a more casual DC fan though admittedly that's grading on a scale with other people who spent time reading comics. I am more of a Marvel guy when it comes to superhero stuff. Most of my recent DC knowledge comes from the animated shows and specials and the movies. If you are a more casual fan then these books are stuffed full of info about the characters of the DC universe and are an interesting resource even if you're not into the game. You're not going to find a bunch of "in issue x this thing happened" kind of references, just a general rundown of the character and some history and noted on gadgets, sidekicks, vehicles, and lairs. My kids spent quite a bit of time just flipping through them and going "hey, that guy!" - so much of these universes are visual that you may not know a characters name but you recognize the look when you come across it - this is where a book has some advantage over Wikipedia. This aspect of it is why I went with books over PDFs - I want books on the shelf my kids can flip through when they're interested. If it was only a game resource for NPC's and villains I find PDFs work pretty well for me, but these are more than that.
There are over 500 characters presented in these two books. Superhero games run off of a steady diet of interesting characters and these volumes provide an immense reserve of characters to draw from. Even if you don't run explicitly in the DC universe there's no reason you can't use the stats and schticks from these to populate your own universe. Change the name and alter the costume a bit - a simple color change may be enough - and you have usable game stats with whatever personality and background you choose to give. It's a very useful set of books for the practical GM.
I hesitate to even call this a review as I really have nothing negative to say about the books. These are good, solid, useful, nice-looking books. If you're a fan of Justice League and the related shows these are cool. If you're a fan of DC in General these are great. If you're running a game of M&M 3 these are as useful a resource as you will find for actually keeping a campaign going.