Well, the classic that still defines this subgenre for me is Anne Rice's 'Interview with the Vampire.' Louis and Lestat's twisted, codependent bond feels deeply transgressive—it's a love story, sure, but one poisoned by immortality and predation. The true 'forbidden' element isn't just human-vampire; it's the monstrous, addictive nature of their connection itself, which the book never shies away from.
A more recent take that grabbed me is 'Empire of the Vampire' by Jay Kristoff. Gabriel de León's doomed love is layered with a grim religious order that forbids such attachments, creating a really effective tension between duty and a love that's literally damned. It's less about sweet romance and more about a love that fuels a centuries-long grudge, which somehow makes the forbidden aspect hit harder.
For something wildly different in tone, there's 'Hot Blooded' by Heather Guerre. It plays with the 'forbidden' angle through a blood debt and a powerful, ancient vampire who's supposed to be her predator, not her protector. The tension comes from that inherent power imbalance and the societal rules of the vampire world she's thrust into.