Most of the books are pretty good and are all worth a read.
I think my favourite is probably The Fall of Reach though, it gives the back story about how the Spartan program started and explains what happened on Reach before it fell. Then I'd go for First Strike, that bridges the gap between the first 2 games and gives a little info on how MC escaped and got back to Earth. He meets up with other Spartans in that one too.
The 2 Forerunner books can be read at any time, Cryptum & Primodrium, to be read in that order. Cryptum is easily the better of the 2. I'd recommend those because they give the backstory on The Didact & his Prometheans. Also touches on the Human-Forerunner war that happened hundreds of thousands of years ago.
There's Grasslands as well, that's s series of short stories but it takes place between Halo 3 & 4. I can't comment on if it's good or not as I'm still to read it myself.