This book reads as though the author wrote down all her ideas for what she'd put in the book...and then stopped. Yes, events happen, but the structure and scenario are forced, the whole middle section reads like a one-act play (all dialogue, no stage direction), and the fantastically written sexual and emotional tension from the first two books is completely gone, replaced by a romance-novel-style third act.
It seemed to me that perhaps the author didn't care to finish the series but had already committed to a contract. Thirds and Fourths certainly weren't as good as the first two, but they were still considerably better than this one. So disappointing.