Late Eclipses by Seanan McGuire

Late Eclipses is the fourth, and most recent at this point, novel in the October Daye series. October's friend Lily, the Naiad, has fallen ill, and no one knows why, or how she may be cured. Shortly after discovering this, and determining to get to the bottom of the matter, Toby is called to the Queen's court, where she is awarded the title of Countess, with the land of Goldengreen, to boot. Toby knows the Queen despises her, so she is certain this honor is really a trap.

Next, Duke Sylvester's wife, Luna, also falls ill during a ball in Shadowed Hills. Toby catches the scent of an infamous and elusive poisoner named Oleander just before it happens, rushes in to discover Luna as she collapses. Toby suspects poison, but many of Sylvester's subjects suspect Toby, especially his daughter Rayseline, who has hated Toby for many years.

Third, the King of Cats, Tybalt, summons Toby to his domain, where he reveals to her that many of his subjects have been poisoned, and some have already died. She begins to work with one of Lily's subjects, who is a chemistry professor at UC Berkley, to try to analyze the poisons used and to concoct an antidote.

At this point, Rayseline brings accusations of attempted murder against Toby before the Queen, and she subjected to a kangaroo court, then sentenced to die for these crimes as well as the previous crime of killing Blind Michael, one of the fae firstborn. Toby's friends break her out of jail, but not before she suffers greatly from iron poisoning from the cell in which she's been confined, and in order to survive, Toby must confront her mistaken assumptions about her past and be changed in ways she never imagined.

McGuire, in her Toby tales, just keeps getting better. The new book is due out in September, and I look forward to it.