The Wings of the Dove
No question that James's prose was a challenge by this time in his career, though I found the later The Ambassadors much easier going and the most compelling of the two books (likely due to my responding to Lambert's age). There's a sort of shimmery pointillist effect with character that James pulls off in TWofD, especially little facets of Kate and Densher as they show themselves in different lights, some particularly unflattering, some very sad. It's all very evanescent, like youth itself.