If Summer Knight was the first "real" book in the series then Death Masks is the book which revealed it wasn't a fluke. Summer Knight told an excellent story about fairies, world-building, character development, and past mistakes. Death Masks, by contrast, is a story which builds on the previous four books to create a story which shows how all of the strangeness so far is interconnected. It's not my favorite of the series but tells a great story while setting up the groundwork for many more adventures to come.
This book follows up on two plotlines established in Grave Peril: the war between the Red Court vampires and the White Council wizards as well as the holy sword wielded by ally Michael Carpenter. The former was left largely off-camera as there was still the possibility of negotiation or turning over Harry to stop the conflict. It's in full-swing now with the White Council doing the fighting for Harry (which I never quite liked).
A representative of the Red Court, Duke Ortega, comes to visit Harry and makes him an offer: settle this in a duel, one-on-one, and end this war now. Harry, justifiably feeling guilty for his role in starting the conflict, agrees and begins preparations for a show-down against an opponent he's woefully outmatched by.
The latter plot element expands on the mythology behind the Knights of the Cross. It turns out three individuals are armed with swords made from the nails in Jesus' cross. Surprisingly, but appropriate given Jesus' views on violence, they aren't really warriors per se but redeemers. It is the job of the Knights to gather up the Denarian Coins (Judas' coins) which cause the holders to be possessed by demons. The Knights of the Cross attempt to save the holders of said coins rather than execute them since each soul lost to the Devil is a tragedy.
I've mentioned my fondness for Michael Carpenter as one of the rare Christian characters in fiction who is neither a caricature or irreligious to the point he might as well not be one. While Harry remains as agnostic as someone can be in a world where the Archangel Michael hands out magic swords, fact he lampshades is ridiculous, Michael remains a beacon of quiet faith. Michael's compatriots play with this idea as one, Shiro, is an honorable warrior but Christian mostly because he liked Elvis' gospel music. Their companion, Sanya, is a black Russian communist who one-ups Harry by being an atheist.
The three of them join Harry in opposing Nicodemus, the leader of the Denarians. Nicodemus is as close to an arch-nemesis as Harry possesses, being a character who has opposed him across multiple books with various degrees of success. Nicodemus is a cunning and evil character with his own sense of humor as well as genre savviness. As impressive as Duke Ortega is, he's in the kiddie leagues compared to the human host of a fallen angel. Death Masks also introduces the Archive, a character who doesn't play a big role in the books but is one of my favorites. There's something awesome about a little girl with near-omniscience.
Death Masks ups the stakes considerably in terms of character danger as deaths are not uncommon in this book. These make the dangers faced by the characters considerably more real and a few of them are unexpected. In short, this book comes with a strong recommendation and it amazes me that it just keeps getting better.