My official recommendations are:

Fall of Reach by Nylund: Read this one, if it's the only one you read.
The Flood by not-Nylund: Don't read unless you can't be bothered to play Halo CE (or the upcoming remake)
First Strike by Nylund: Not as good as other Nylunds, but still pretty good. Pretty much required if you want to read:
Ghosts of Onyx by Nylund: No Master Chief, but full of characters from other Nylund books. Has a lot of back story that might influence how you view Halo 3.
Journal of Catherine Halsey (included with high dollar copies of Halo Reach) by Nylund: Short, and more than a little meta, but a very good read.

Contact Harvest by not-Nylund: Don't read, unless you can't get enough Sgt. Johnson. Also features a Grunt as a main character. It's actually written by Joe Staten, Bungie writer and voice of the Grunts.
Cole Protocol by not-Nylund: I couldn't get through more than the first couple chapters.
Halo Evolution vol 1&2: I don't know... But I'd like to read them.

Cryptum: Forerunner #1: by Greg Bear: Sounds good?

Halo comics- I haven't read any that I'd recommend.
Halo Legends- Pass

Notice any trends?