Are the books still worth checking out then? I know my friend has them, I could borrow them if they are good.
How actively involved was Bungie in the books?
They're entertaining. I don't know that I could recommend them, given my opinion of where the story ended up with the games, but whatever. Bungie was involved in story approval and such, although I'm not sure if they guarded it down to the minute detail. (They certainly didn't seem to remain consistent with the details of them.)
The books go Fall of Reach, The Flood, and First Strike. (Before, during, after Halo: Combat Evolved). The Flood is written by a different author and is not quite as good, considering a large bit of it just details what you do during the game. There are other vantage points introduced (focusing on the surviving humans and a ill-matched Elite and Grunt, so those parts are enjoyable.
There are other books, too. Ghosts of Onyx takes place after First Strike and during Halo 2, but does not really have anything to do with the game. Master Chief is not a central character. Contact Harvest takes place before Fall of Reach, and I have not read it.
Fall of Reach, First Strike, and Ghosts of Onyx are all written by Eric Nylund. I kind of wish they gave him the keys to the universe after Halo 1 and Bungie never touched it except for multiplayer.