The Lyttons seriously lost their luster by Book 3. :( Into Temptation lost my interest pretty early on and became a chore to slog through. Everyone's shenanigans felt trite and redundant.
I really enjoyed the first book in this trilogy. No Angel was an engrossing and layered, period family saga. Book 2, Something Dangerous took the family through WW2 and was slightly less engaging and felt overlong. But Into Temptation was just plain disappointing. There was no longer a strong character to like, root for...or even respect! Barty turned into such a disappointment! I loved her from the first book and thought she would be the Celia Lytton-like character through the end, but instead she was turned into a rather limp noodle.
The only other likable characters were Oliver Lytton who died in Book 2, and some of the American contingent like Jamie, but even he was rendered largely impotent to the plot in favor of his most unsavory brother Lawrence, who unfortunately ended up being a major thread throughout this final book, it was SO disappointing--he was like a boring and haunting demon. It's not like he had had any redeeming qualities to speak of. Why was he given so much significance here? I'm still yawning.
So if you're a fan of the trilogy, you'll definitely have to finish. Many others enjoyed all three books, but for me the trilogy fizzled.