Kay is probably the most underrated fantasy author. Tigana is among his best and it may be the best stand-alone fantasy novel ever.
For your MiL, I would recommend The Tourist by Olen
Steinhauer. It is widely regarded as one of the best spy novels released in the past decade but, despite that, I know few people who have read it. If she enjoys it, there's also a sequel. Edit: two sequels now, apparently. I need to get reading.
If you want to go the mystery route, I'd recommend anything by Rovert Goddard. He's an excellent British writer with something close to 20 novels. It's unlikely that your MiL has read him because those books have only recently come into print in the US.