This comes to mind
War for the Oaks by Emma Bull
https://www.goodreads.com/book/show/...r_for_the_Oaks
Winner of the Locus Award for Best First Novel (1988)
-main female character
-faerie world mixed with our world
-romance sub-plot
-the most difficult point: a good sub-plot romance where the female protagonist at the beginning doesn't like and/or trust her future lover/partner
(this is present too)
-the heroine is in her 20s
Synopsis:
Eddi McCandry has just left her boyfriend and their band when she finds herself running through the Minneapolis night, pursued by a sinister man and a huge, terrifying dog. The two creatures are one and the same: a phouka, a faerie being who has chosen Eddi to be a mortal pawn in the age-old war between the Seelie and Unseelie Courts. Eddi isn't interested--but she doesn't have a choice. Now she struggles to build a new life and new band when she might not even survive till the first rehearsal.