Previously read half of these. This is for the ones I haven't read.
Demonology: Nice look at Adam's mom Talia and her relationship with Adam. I loved her! She is very supportive of Adam. One of the few humans aware of the supernatural.
Chivalrous: Reece's backstory. Tragic. Betrayal. Heartbreaking.
Lucifer's Daughter: Hope and Karl. Sweet and romantic.
Vanishing Act: Savannah and Adam. I liked how they talked (after hitting some bumps). Savannah has really matured through this series.