What is meant by a "teacup mystery" book?

Answer 1

I think it means that it's more of a "cosy mystery," than violent or gruesome.

Answer 2

I found this:

The teacup mystery is solved when drinks are ordered—they’re meant to hold cider, or “cidre” as it is spelt, French style, on the menu. Cider is the standard drink in creperies, and there are several different types on offer here. It’s worth eschewing the rather obvious choices for the more unusual flavour of the Poire de Normandie. As its name suggests, Poire de Normandie is a pear cider, and is light and sweet without any of the acidic aftertaste that so often accompanies similar drinks.