I made the mistake initially with the Dream-hunter series as thinking of them as a separate series to the Dark-hunter books. They're really not. You lose some important elements of the world if you don't read both strands, and let's face it, the Were-hunters aren't hived off as a separate series. I reckon the good guide is that if Acheron or any one of his connections shows up, it's really part of the same series, and Acheron does make a significant appearance in this story.
You see the crossover clearly in the opening scene where our heroine, Simone is at lunch with Tate - the New Orleans city coroner we first met in Night Pleasures, rescuing Kyrian and Amanda from the daylight. The hero, Xypher, we have already met too - in Devil May Cry where he helps out Kat and Sin, and earns his time as a human, which is what we share here.
The strongest connection plot-wise is undoubtedly with Devil May Cry, as we're awash with demons, particularly the Gallu we first met in that book.
Simone teaches future pathologists, hence her connection with Tate. Her choice of career is driven by the tragedy of her childhood.
We also, because of Simone's unique talent, get to meet some ghosts. Most importantly Jesse the young teen trapped in a 1980's timewarp that those of a certain age will remember vividly. He's great fun, and adds lightness and humour as another struggle between the defenders of humanity and those who want to rule the world at any cost ensues.
We also get to meet Jaden - the demon broker, a character it's worth taking note of for future reference. He sounds like someone who really deserves his own story, for sure.
The Simone-Xypher story is one of discovery. One discovers their humanity, and the other discovers something unique about their heritage and learns to embrace it. Together they find love and a future together.