Most of the BPRD graphic novels to date have worked relatively well as stand-alone stories, with certain character revelations being introduced but the plots being decipherable (well, as much as they ever are) without a lot of familiarity with the earlier collections. Not so with this 10th collection in the series, which announces itself as the first book in the Scorched Earth trilogy.
The Warning tries to bring people up to speed, but I think it would be pretty confusing for those who haven't read several of the earlier graphic novels, such as Plague of Frogs or Garden of Souls. You've got legions of evil frog-beasts, strange devolved hominids building vast machines in the bowels of the Earth, visions of Cthuluesque monstrosities taking over the world, mysterious sorcerers appearing and disappearing in astral form, portents of a big destiny for Liz Sherman that belies her seemingly humble origins, and cryptic messages from the spirit of Lobster Johnston.
The city of Munich gets largely obliterated, the BPRD troops get some more serious gear, Liz gets kidnapped again by occult entities, and Abe Sapien starts acting exactly like Hellboy. Seriously, I don't know if the authors realize this, but in this storyline the personality transformation that began in Garden of Souls seems to be pretty much complete. Abe is talking and acting just as gung-ho, aggressive, and careless as Hellboy here. Still fun to read, but it just made me miss Hellboy actually taking part in these stories. I liked Abe more when they had more differences.
Overall, another good graphic novel, setting up a story arc that looks to deal out tremendous mayhem to Earth as ancient powers seek to reclaim their power and transform the planet into their playground. I'd be surprised if Hellboy himself doesn't make an appearance of sorts at some point in the process.