The best place to start reading X-O Manowar is from the beginning of the Valiant reboot at issue #1. The first trade paperback of the reboot is X-O Manowar: By the Sword which collects issues 1-4.
The sequel volumes are also recommended and go as follows:
- X-O Manowar: Enter Ninjak (issues 5-8)
- X-O Manowar: Planet Death (issues 9-14)
- X-O Manowar: Homecoming (issues 15-18)
- X-O Manowar: At War With Unity (issues 19-22) & Unity: To Kill a King (issues 1-4)
If you want to read some of the original, classic X-O Manowar then consider picking up X-O Manowar: Birth (issues 0-6).