Help! Really struggling with a subplot.

I wonder if any readers out there could recommend any novels with subplots which they think fit easily into the storyline and add another layer of interest without slowing the plot down?
I think Kate Morton in the House at Riverton does this brilliantly. Any other suggestions would be wonderful. Just so I can read some novels the book club community rates and and study the different techniques other writers use.
I'm finding this tricky.
Many thanks
Judy

Help! Really struggling with a subplot.