Reddit mentions of Book of a Thousand Days

Wintergirls by Laurie Halse Anderson, definitely. It's also about mental illness and eating disorders, but toxic friendship is central.

In a MG vein, Gail Carson Levine's Fairest and Shannon Hale's The Book of a Thousand Days also have a toxic friendship element, though in both cases it's muddied by the power differential between mistress and servant.

Gregory Maguire's already been covered, so most of these are middle-grade or YA titles.

• Ash is a lesbian version of Cinderella.
  
• The Goose Girl is a retelling of Grimm's version of... The Goose Girl.
  
• Zel is Rapunzel in 16th-century Switzerland.
  
• Breath is the Pied Piper + cystic fibrosis in the Middle Ages.
  
• Bound is a retelling of the original (Chinese) Cinderella story.
  
• The Book of a Thousand Days is Maid Maleen in the Caucasus.
  
• Shadow Spinner is Arabian Nights from a different perspective.