Late Eclipses by Seanan McGuire (October Daye #4)
Synopsis from GoodReads:
October “Toby” Daye, changeling knight in the service of Duke Sylvester Torquill, finds the delicate balance of her life shattered when she learns that an old friend is in dire trouble. Lily, Lady of the Tea Gardens, has been struck down by a mysterious, seemingly impossible illness, leaving her fiefdom undefended. Struggling to find a way to save Lily and her subjects, Toby must confront her own past as an enemy she thought was gone forever raises her head once more: Oleander de Merelands, one of the two people responsible for her fourteen-year exile.
Time is growing short and the stakes are getting higher, for the Queen of the Mists has her own agenda. With everything on the line, Toby will have to take the ultimate risk to save herself and the people she loves most—because if she can’t find the missing pieces of the puzzle in time, Toby will be forced to make the one choice she never thought she’d have to face again…