Dianne Duvall's Immortal Guardians series has three books so far, plus a novella included in a collection called Predatory. The fourth book comes out later this year. I picked up the first book on a whim and was immediately struck by the complex world created in this story. There are normal humans, obviously. There are also "gifted ones," or humans with supernatural abilities and "advanced" DNA. But vampirism is completely unrelated to the abilities of the "gifted ones." In fact, it comes from a virus that turns a human into a vampire and turns a "gifted one" into an immortal. The thing about the vampires is that they tend to go crazy after a few years. Crazy as in "homicidal." So the job of the immortals is to hunt down the vampires and stop them from killing people. The immortals, due to their already differing genetics, are protected from the insanity that eventually kills any human-turned-vampire.
Then we have Ami and Zach, who are...none of the above. We don't find out Ami's secret until halfway through book 2, and still don't have an explanation for Zach. Though to be fair, Zach has only shown up in a few scenes, and we don't even learn his name until the third book. Usually a world that starts out with vampires will add other supernatural elements, such as werewolves, demons, and magic. Duvall takes the scientific approach instead, which I find pretty cool.
So we have complex characters. We also have tons of action. There is no shortage of fighting, death, and bloodshed. We get descriptions of all of the characters' weapons. The male main characters are all Immortal Guardians, so they have centuries of experience with fighting and killing vampires. But the women are no slackers, either. Sarah (from book 1) is a college professor who is thrown into the world of immortals and vampires by accident and is more than capable of holding her own. Ami (from book 2) kicks some serious vampire ass, despite being tiny and seemingly vulnerable. Think River Tam with Merida's red hair. And Dr. Melanie Lipton (from book 3) is smart and fearless. She knows what she wants and isn't afraid to stand up for the vampires she's trying to save. Makes me wonder if Duvall is a fan of Buffy herself.
There is just so much I love about this series. The characters are well-rounded, and Duvall does a great job of dropping subtle world-building hints and raising questions that are left unanswered at the end of each book. At the same time, the main story itself in each book is resolved so that each novel stands on its own. And I'm not gonna lie, the steamy scenes are pretty good.
Oh, did I mention these are paranormal romance novels?
Nine times out of ten, you would not find me defending bodice rippers. Usually the plot is thin, the women need saving, and any "foreshadowing" is just setting up two minor characters for their own love story. If you remove the relationship between Alice and Bob, most romance novels fall apart completely. I've read a few Harlequins in the past, and they were all pretty forgettable. The Immortal Guardians books are the first romance novels I've read (primarily) for the plot, and actually cared about the overall story.
Make no mistake, the Immortal Guardians books are romance novels, complete with naked torsos on the cover. If you removed all traces of romantic relationships, admittedly, the series would be a lot thinner. Still, that's true of nearly any series. If you have enough characters, eventually some of them are going to get together (in fanfiction if nowhere else). But you could remove all of the sex scenes and still have an exciting, compelling story in a complex world. Which makes me wonder why they were written and marketed as romance novels in the first place.
The one quibble I have with the series is a minor one: all of the immortals are literal vegetarian vampires. They need blood, but they also eat normal food, and they are all health food nuts. We get loving descriptions of vegetarian meals, prepared with organic ingredients. But it never rises to true Author Tract levels and the story is still definitely enjoyable even if you don't happen to share her passion for salads.
Overall, I would give the series 4.5 out of 5 stars. If you like action, supernatural creatures, and a dash of hot romance, definitely check it out.