As WP:60 discussed in Week 40, Wikipedia articles need citations. That way, readers can verify the information is legit. But just because Wikipedia is online does not mean that the encyclopedia must rely solely on online content.
Dust off those books, journals and other paper sources; the information contained within is just as fitting for Wikipedia as any online source. (Remember: History existed before the internet!) In fact, as the essay Offline sources notes, using offline sources has its benefits. Relying solely on online materials sometimes leads to recentism, the imbalance in articles tilting toward recent events.
If you use offline sources, be sure to fill out its bibliographic information as much as possible in citing the literature. That helps other Wikipedians locate the referenced material should they need it. You can also add a quote within the citation to show readers exactly how the cited text reads.