Dark Avenger’s Sidekick book 3, with spoilers ahead for the earlier books. Also Moth & Cobweb 6, with milder spoilers for The Green Knight's Squire.
It opens with Yumiko brooding over what she has learned of: her mother's murder, who had kidnapped Elfine, her missing beloved Tomorrow Moth, and that his enemies planned to have Sir Garlot murder Sir Gilberic in a fight. She pulls herself together and listens to the bugs she planted -- learning more.
Sending her off on a tale that involves visiting Winged Vengeance's lair, talking with the dog Ruff and concluding she has to tell him things in good faith to win his trust, a treasure Yumiko's mother gave her before she died, learning someone can turn into a bat, discussing family relationships, a woman madly in love demanding her husband's freedom, a discussion of mercy, fighting werewolves, and more.