God Stalk
A dark, inventive fantasy with mesmerizing worldbuilding and a wonderfully strange heroine starts incredibly strong, only to squander its promise on meandering plots and a disturbing relationship involving a child murderer. I loved it until I very much didn't.
The Armies of Daylight
I stuck with Barbara Hambly's Darwath trilogy out of love for the author, only to find an ending that made three books of struggle feel pointless. Flat protagonists, frustrating plotting, and one of the most unsatisfying finales I've ever read.