Wikipedia's reliable sources guideline states that articles should be sourced with reliable, third-party, published sources. Even though Wikipedia is an online encyclopedia, there is no distinction between using online versus offline sources.
While many editors use online sources, such as websites and online journals, many great sources are only available offline in printed books and paper journals. Don't let the fact that a printed book or journal is not available online scare you away from using them as a source in Wikipedia. Likewise, do not remove cited material merely because it is from an offline source.
That Wikipedia relies extensively on online sources is not surprising, considering the relative ease of accessing such materials. There is also an additional advantage of using online sources, because it allows all users to evaluate the source and its value to the article. However, this reliance on online sources can lead to recentism
, where most articles and content are from the Internet era
. It can also lead to an unfair bias against print books and print journals, where an editor's addition of material sourced from a book or print journal is reverted with the comment "Revert - I couldn't access and confirm this source online."