The Flood also has a couple ODST segments if I recall correctly, but it is the most boring novel if you played Combat Evolved. It is just an adaptation of that game.
If you want a great ODST story, pick up Halo Evolutions. One of the short stories in there is about an ODST, and it is amazing!
Also, I like Cole Protocol, though it was different than the other stories. Not for everyone, but not bad.
I still need to get Cryptum, and then eventually update to the newer versions of Evolutions. I'm going to skip the rereleases of the original 3, since I heard that nothing was really changed.