Tentatively:

Les Miserables - Victor Hugo: Dahlia/Zephine, Dahlia/Favourite, Baptistine/Magloire, Zephine/Fantine, Fantine/Simplice (I could swap Eponine/Cosette for Fantine/Simplice if anyone is likely to request them for the book--I notice they were already nominated in All Media Types and guessing that nominator was thinking more musicalverse where Eponine/Cosette is really the only possible pairing without time travel incest and probably wouldn't want to be stuck having to offer minor book character pairings...)

Tortall - Tamora Pierce: Buri/Onua, Yuki/Haname, Delia/Alanna, Delia/Josiane, also thinking about Beka Cooper/Okha Soyan (I think Okha is pretty clearly a trans woman despite the consistent use of male pronouns by other characters--I can dig up author interviews to that effect, although I do not endorse Pierce's rationale for it)

Romeo and Juliet - Shakespeare: Juliet/Rosaline, Lady Capulet/Lady Montague

Detective Dee and the Mystery of the Phantom Flame: Wu Zetian/Shangguan Jing'er

Phantom of the Opera - Gaston Leroux: Christine/Meg, Carlotta/La Sorelli, (happy to add others if people can think of them)