The trouble with 2008 Guardians (awesome though that series may be) is how tightly interwoven it is with the other Marvel Cosmic arcs. You can't really read Guardians without first reading Annihilation and its associated tie-ins, Annihilation: Conquest and its associated tie-ins, and then when Guardians actually starts, you have to read it as part of War of Kings, which happens to bring in X-Men stuff on top of the fallout from Annihilation: Conquest...
Here's the best reading order I can give you, using trade paperbacks:
Annihilation
- Thanos: Redemption
- Annihilation: Book 1 (includes Drax: Earthfall, Annihilation Prologue, and Annihilation: Nova)
- Annihilation: Book 2 (includes Annihilation: Ronan, Annihilation: Super-Skrull, and Annihilation: Silver Surfer)
- Annihilation: Book 3 (includes Annihilation and Annihilation: Heralds of Galactus)
Annihilation: Conquest
- Annihilation: Conquest - Book 1 (includes Annihilation: Conquest Prologue, Annihilation: Conquest - Star-Lord, and Annihilation: Conquest - Quasar)
- Nova vol. 1 - Annihilation: Conquest (Nova vol. 4 #1-7; note that#4-7 are also included in Conquest Book 2)
- Annihilation: Conquest - Book 2 (includes Annihilation: Conquest - Wraith, Nova #4-7, and Annihilation: Conquest)
- Nova vol. 2 - Knowhere (Nova vol. 4 #8-12 & Nova Annual #1 - NOTE: I recommend reading in this order: Annihilation: Conquest #1 and 2, then Nova #8-10 and Nova Annual, Annihilation: Conquest #3-5, Nova #11-12, and finally Annihilation: Conquest #6, in that order)
- Guardians of the Galaxy, vol. 1: Legacy (Guardians of the Galaxy vol. 2 #1-6)
War of Kings
I'm sorry, but this is when things get confusing, as X-Men and Inhumans stuff rolls into the picture now...and this is also what Guardians is most tied in with. So hold onto your butts, it's gonna get a little bumpy:
- Nova vol. 3 - Secret Invasion (Nova vol. 4 #13-18)
- Secret Invasion: Inhumans
- X-Men: Deadly Genesis
- Uncanny X-Men: Rise and Fall of the Shi'ar Empire
- X-Men: Emperor Vulcan
- Nova vol. 4: Nova Corps (Nova vol. 4 #19-22; but don't read issue #22 just yet!)
- Guardians of the Galaxy, vol. 2: War of Kings, Book 1 (Guardians of the Galaxy vol. 2 #7-12)
Now's where it gets really complicated. Here are the relevant trade paperbacks for the rest of War of Kings:
Problem is, these intertwine so much that you need an issue-by-issue breakdown for those four trade paperbacks, otherwise you'll be like me, slowly realizing that you're just re-reading the same big climax from everyone's perspective, and it doesn't build any more from there like you'd expect it to.
Realm of Kings
Thankfully, this one's much easier:
- Guardians of the Galaxy vol. 4: Realm of Kings (Guardians of the Galaxy vol. 2 #20-25)
- Realm of Kings
- Nova vol. 6: Realm of Kings (Nova vol. 4 #29-35)
And then they get even easier, because now they're just single event TPBs instead of a bunch of crazy War of Kings shit:
The Thanos Imperative
Annihilators
Annihilators: Earthfall
And then some shit went down between the Avengers and the X-Men, and some other stuff happened, and then Bendis took over the Guardians and I stopped reading. These are part of the whole "Marvel NOW!" thing, and as such they were meant to be a jumping-on point for new readers:
Guardians of the Galaxy vol. 1: Cosmic Avengers
Nova vol. 1: Origin
Guardians of the Galaxy vol. 2: Angela
Nova vol. 2: Rookie Season
Frankly, I wasn't a fan of Bendis' writing for the Guardians (especially Rocket), so I didn't keep up with those.
--EDIT--
I did just find this pared-down list that focuses on Guardians characters, but I think it'll be somewhat confusing, especially the further into things you get. But if you don't want to read EVERYTHING, you can try this:
- Annihilation: Book 1 (Drax the Destroyer and Nova)
- Annihilation: Book 3 (you can skip Heralds of Galactus)
- Annihilation: Conquest - Book 1
- Nova vol. 1 - Annihilation: Conquest
- Annihilation: Conquest - Book 2 (you can skip Wraith and the Nova issues, which are also in Nova vol. 1)
- Nova vol. 2: Knowhere
- Guardians of the Galaxy Volumes 1-5 (issues #1-25)
- The Thanos Imperative
- Annihilators
- Annihilators: Earthfall
- Guardians of the Galaxy (Marvel NOW! reboot)