Incubus. Coven, #12

Eat your heart out. When Kathy meets Charlie at the library, the two have instant chemistry. The only problem is that he's quite a bit older than her. Despite that, Kathy finds herself falling for Charlie. Hard. Meanwhile, Samantha is wrapped up in her own marital woes that distract her from noticing just how much Kathy is giving up for her new boyfriend. When she finally does notice, she discovers that Charlie is actually an incubus who intends to impregnate Kathy and take her back to his coven to bear his children.
The only problem is that Kathy is already under his spell. Incubus is the twelfth book in the Coven series, which is part of the Art of Magic universe, containing the Lost By Magic and the Under the Moon series.