I expect this is because at this point in the series there is so much already in place that adding more because redundant as well as difficult to squeeze in between all the back story -- there's the Dirk and Steele detective agency itself, with its reclusive telepath chief and its alliance with the hidden family of magicians against the evil family branch of magicians called the Consortium, and there are the gargoyles with the elaborate loyalties and fears of witches, and the various shape-shifters, and I don't think the mermaids get mentioned at all this time, and, and and. So the new dangers of blood-seeking demonspawn barely gets an eyebrow raise.
I felt a bit disconnected from Eddie -- I vaguely remembered him from previous books, where I think he was a strong secondary character, but not enough that the outlines of his current super-angsty position made much of a connection. And Lissa never really seemed real -- the poor girl of a Romeo & Juliet style marriage who is shunned by friends and stalked by foe -- oh well. It's all right for fans of the series, but I don't recommend it as an entry point. I put up with their mournful and lost glances at each other because I felt confident that something crazy would happen soon, but new readers may not have the patience.