Deliverance Lost concerns the Raven Guard, who were a loyalist legion of Space Marines who were almost wiped out several books ago. This novel takes place just after the end of both Fulgrim and The First Heretic, because the Horus Heresy series has a complicated structure, often looping back on itself.
The Raven Guard are meant to be a stealthy, sensible legion of Space Marines, rather than the usual bunch of tools (sorry, "strategic geniuses") like the Emperor’s Children who sprint at their enemies and can’t work out why they take casualties. They’re also a bit obsessed by crows.
At the start of the novel, they’re on the run; there are only a few hundred left, and they’re bsuy being wiped out by Horus’ forces. Feeling glum, Corax, their primary, fancies dying in a last stand, but then sensibly decides to retreat when some of his support crew fly in to rescue him.
The rest of the novel concerns how Corax tries to rebuild his legion and get revenge, and how he’s stymied. First, Corax has to sneak his spaceship away from Horus’ forces. Then he goes to earth, battling various flashbacks about where he grew up on a moon called Deliverance, and then gets in an argument with Rogal Dorn, the strangely named master of a different legion, who wants him to hang around on Earth waiting for the inevitable assault.
Corax is having none of this and persuades the Emperor to give him a tool to mass produce new Space Marines, and takes this MacGuffin back to Deliverance to start the counter attack.
Unfortunately, in an en masse version of John Woo’s Face/Off, the Raven Guard have been infiltrated by another legion, the Alpha Legion, who sneakily used surgery to steal the faces of dead Raven Guard legionnaries. The rest of the book, then, follows the sabotage of the Raven Guard’s revival and a battle on Deliverance, but while at the end of this Corax has suffered numerous setbacks, he’s still ready for more fighting.
It’s not a dreadful volume in the general scheme of things, but again with Corax not being the most interesting of characters, and the writing being a bit overblown but without any fun existentialism thrown in, I found it a bit wearing. I assume there’s more of a pay off later on.