。
The question is where to buy that book.
Literal
Problem [topic-は] where [at-で] that book [obj-を] buy [Q-か] [copula-です].
An embedded wh-question — どこで買うか ("where to buy") nominalized by か, equated with the topic ("that book"). The polite copula です marks the formal register. A common framing for posing a problem or question for discussion.