Black Library have released the Dan Abnett book Double Eagle as an eBook.
From their website:
While the Space Marines and Imperial Guard rule the battlefields of the Sabbat Worlds, the skies belong to the Imperial Navy. On Enothis, the enemy are threatening to halt the Imperial advance. Daemonic forces push into Imperial territory, thousands of refugees flee across the deserts and an infamous Chaos-tainted pilot, Khrel Kas Obarkon, rules the sky, his skill in the cockpit matched only by his kill tally. The ace pilots of the Phantine Twentieth Fighter Corps are the Imperium’s only hope for victory.