The books were originally published in 1999 but have since been reprinted by Crossroad Press as part of a deal with Paradox Interactive (new owner of the White World roleplaying game company). This was my favorite of the books originally but my tastes have changed a bit in twenty years. Some things I enjoyed about the book earlier like the action are now less important than the character-building while I now wish for more focus the lead's motivations. Something I feel this book was a bit lacking on.
Lasombra features the arguably most popular of all Vampire: The Masquerade characters, Lucita of Aragon. A literal Spanish Princess from the Dark Ages, Lucita was Embraced by her disgusting sire Moncada in order to satisfy his lusts. Moncada was a holy churchman as well as a vampire and Lucita found herself rebelling against him as both sire as well as spiritual leader. Centuries later, she earns her living as a paid assassin of the undead.
Lucita is hired by a mysterious employer (possibly multiple mysterious employers) who wishes her to kill a number of high-ranking Lasombra agents crucial to the military efforts of the Sabbat. She is also supposed to kill one of the three Archbishops that will cripple the invasion of the East Coast by her clan. Lucita has no loyalty to either her Clan or the Sabbat and mostly just wants to collect her expensive fee.
Much of the book is devoted to the lengths the Camarilla is willing to take to slow the Sabbat's advance on Baltimore. They start abandoning their less valuable cities, mass Embracing cannon fodder and leaving them to die despite this being against their cardinal beliefs. Theo Bell and the Princes are utterly unrepentant about this, considering their victory worth any costs. They also move away anyone "important" among their kind.
On the Sabbat side of things, we follow an ambitious young Pack Leader named MacEllen who is eager to prove himself as a potential leader of the sect by completing impossible orders. MacEllen believes he's being sent to his death, unaware the Sabbat is aware of all the Camarilla's secret movements due to a secret Lasombra among their ranks. The most important character in the book, even more than Lucita in some ways, is Talley the Hound who is the Sabbat's greatest assassin and reluctantly assigned to stop but not kill Lucita. This is on the order of Moncada, her lecherous and vile sire who is now a high-ranking member of the Sabbat.
If the story sounds convoluted, it really isn't as it progresses from point to point and allows you to soak up in the complicated power games between the two sects. In many ways, it is a much more Jyhad-esque (what vampires call their political games before the word would become uncomfortable to use due to the War on Terror) than the Ventrue Clan novel. Games within games, wheels within wheels.
There's some really good moments spread throughout the book like a young Nosferatu Neonate's giddiness at the opportunity to Embrace and torture a dozen handsome young men like the kind she used to hang out with, Talley's world-weariness at having to deal with so many idiots in the Sabbat heirarchy, and Moncada's depraved paternalism to Lucita despite how thoroughly she's rejected him. This is in-between some really well-written action sequences that incorporate vampire Disciplines, emotion, and twists.
Unfortunately, the biggest weakness of the book is Lucita herself. For a character so beloved, she spends most of the book in a state of perpetual anger and disdain for everyone around her. There's not much character development for her and while she does a large number of impressive stunts, we don't get much of her character other than, "Don't piss her off." Talley, her opponent, actually gets the meat of the character development.
Lasombra is an okay installment in the series but not up there with Setite or Gangrel. Both of those books heavily focused on the character development of its protagonists. Even Ventrue had a lot more insight into who its main character was. Lucita mostly remains a mystery but she sure does look cool killing people.