Nine Perfect Strangers by Liane Moriarty

Novelist Frances Welty is hoping that time at Tranquillum House will provide her with the rest and relaxation she needs after her latest book has been rejected for publication. The other eight guests are also hoping for time away and a transformative experience, but they worry about the rules--giving up your cell phones and having to eat very healthy food for the ten days they are there. Meanwhile, owner of the spa, Masha Dmitrichenko, and her employees, Yao and Delilah, are set on providing a memorable time for everyone--just not in the way the guests have in mind...