A few more books that fit the tone of Girls Underground but are missing some crucial elements.
Riddle of the Wren by Charles de Lint
Minda, 17, lives a mundane life, her mother dead, her father no comfort. But she is haunted in her dreams by an evil creature named Ildran. One night, in the dreamworld, she meets Jan, who both initiates her into this new life by giving her special tools and protective talismans, and asks for her help.
Minda sets out on her journey and immediately loses her way, ending up in a different world. She finds companions, including an animal, and fights the minions of the adversary. Going from world to world, she does battle, manages to rescue Jan, and learns she is More than she knew. She faces the adversary alone, and reveals the fraud behind his power.
Under the Green Hill by Laura L. Sullivan
Meg, a teenager, is sent with her siblings to live with old relatives in England when an illness sweeps through her own country. Strange rules are in set down for them immediately, which they mostly ignore. But the danger is real, for faeries are abroad and it is time for a battle in which humans must fight.
Guided by a boy named Gul, the children meet the Seelie Queen and quickly become entangled in faerie business. Meg’s brother volunteers to fight in the battle, and she must rescue him from this fate. But the situation is complicated, and there is no single clear adversary. Still, in the end, Meg understands she is More than she thought, and finds a courage she never expected.
Runemarks by Joanne Harris
Maddy, 14, lives in a world where magic is feared and reviled, but she has had certain abilities since she can remember. Her only companion and confidante is a traveller named One-Eye who comes to her village every year and teaches her to use her powers. One day he asks her to retrieve a special item for him in the world below, and opens a door for her to go underground, past the realm of goblins (where she temporarily receives some assistance from a reluctant goblin). She finds the object but cannot get it yet, and in the meantime befriends a man named Lucky who turns out to be more than he appears to be.
Maddy ends up on an epic journey, meeting gods and goblins and men, all in an attempt to save her friend One-Eye from a prophesied doom. Her adversary is unclear at first, but is eventually revealed to be the Nameless, a god who wants to destroy all the worlds. She must go to Hel to fight him – but she doesn’t really end up defeating him personally. She does, however, discover she is More than she ever imagined.