Barachiel picks up the story where the Book Of Raziel -- the book kept by the Angel who stood by the side of God's throne -- is being brought into Alexandria, and it's hard to imagine just how much story and action Leon packs into such a short story; I read it in about 15 minutes and it had as much going on as any novel might, in that time; it just tore on through it.
I've said before and I'll say again: Leon's stuff deserves to be more widely-read, and would be great turned into movies or television shows; in the past he's done Spielberg-esque modern fantasy and then moved into stranger and darker stories, and this series feels like the culmination of what he's been heading for: a dark, strange, mysterious, epic story that combines fantasy, history, and religion into a heady mix.
What I particularly like is the realistic feel to it; so many religious stories feel sanitized, like they're middle-school pageants. Leon makes angels feel real the way Christopher Nolan made Batman and the Joker feel real: he creates beings and a world around them that you feel could exist (or maybe did). The angels and the humans are full of passion and energy, but even better, Leon makes the angels different. They aren't just humans with greater powers: his angels think and act differently, have different motivations and different passions than humans. Too often, people write superheroes, angels, wizards as though they were just humans with some neat tricks. But an angel wouldn't think or act like a human; they're a different creature altogether, and these books highlight that.
I'll be reviewing them all over time here, but you shouldn't wait: go start the whole series now, with Asbeel and work your way through them.