Best Halo books?

I'm just finishing up First Strike and beginning Ghosts of Onyx and I wondered what other Halo books are worth checking out. I'm primarily interested in books focusing on Spartans (duh) but I do find the entire Halo universe to be very intriguing so well-written books about the Covenant, the Forerunners or the UNSC in general would be cool to look into.

Are there any exceptionally good ones I should make sure not to miss? I'm not typically into graphic novels or comics and it's my understanding a sizable portion of the Halo expanded universe material is of this nature, but if they were truly exceptionally good I'd be open to checking them out. Thanks for any opinions you can offer!