The Woman in Cabin 10 by Ruth Ware

It's a mystery. Lo is sent on assignment to cover a new kind of luxury cruise ship for a travel magazine.  She has just had her apartment robbed.  She is a bit paranoid, and a lot dramatic.  She thinks the whole luxury ship thing might be just a bit off.  It gets even worse when she believes a woman that no one else on the ship seems to believe exists falls overboard.  It's a mystery