。
Please don't put the books in upside down.
Literal
Book [object-を] upside-down-[adverbial-に] put-[please-don't-ないでください].
さかさま ('upside down, inverted, backwards') is a な-adjective/noun; the adverbial form さかさまに modifies the verb. ~ないでください ('please don't do X') is the negative polite request, formed from the negative te-form ~ないで + ください.