Symbolism, Imagery, Allegory
You better believe that blood's going to play a pretty critical role in a book called Hold Me Closer, Necromancer, and it definitely makes a huge appearance in the story. Douglas drains some of Bridin's blood for his experiments, Brooke gets dismembered, there's major necromancer-on-werewolf carnage later in the book, and Ramon becomes a hybrid were-bear as a result of contact with bear blood. Even though it's billed as a young adult novel, this is still a pretty violent book.
We could say a lot about each of these individual incidents of bloodiness, but the most important overall symbolic punch that blood packs is that it's the key ingredient for power and control, which is a major theme of the book.
Take Sam's necromancer lessons, for instance. Ashley tells Sam that while his own power should be enough to raise the dead, a blood sacrifice is necessary for maximum efficiency. It "depends on the necromancer," she explains. "A strong one can get by with very little blood. He won't need as much of a power boost, but the offering should be there" (21.23). Sure enough, when Sam uses his own blood in the ritual, he's able to raise Ed, a higher-level zombie entity, from the dead, which kind of ticks Douglas off.
Blood is more than just a life force in this book. It's directly connected with power, over the living and the dead alike. So while it's not a symbol for the squeamish for sure, it gets the job done in Sam's battle to defeat his enemy.