|This page in a nutshell: Offline sources are just as valid as online sources.|
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."
Books are a typical example of an offline source. These are often great resources for history, philosophy and literature, and they often contain information that can't be found online. Several ongoing projects, such as Project Gutenberg, Internet Archive, NLA Trove and Google Book Search, aim at digitizing certain books or newspaper articles and presenting them online. Even if the books are online, it might be necessary to consult a print edition to double-check any errors from the OCR scanning.
Many academic journals only make short abstracts available online. Other content providers, like the Wall Street Journal, publish their content behind a paywall that prevents non-subscribers from accessing the content. Other websites, like the Philadelphia Inquirer, only publish their content online for a few weeks. Sometimes a source was once online, but now is offline (link rot).
Special care should be taken when using offline sources. Provision of full bibliographic information helps Wikipedia's readers and editors find the source when they need it, and also increases the source's credibility as a reliable source. This is often done by using a fully-filled out citation template such as ((cite book)) or ((cite news)). Use of the
quote= parameter within those citation templates provides some context for the reference. This is especially important when using the off-line source to support a fact that might be controversial or is likely to be challenged. Providing identifiers such as an ISBN, OCLC number, Open Library number or similar can help others locate physical copies, as cataloguing data can often vary from one library to another.
Many offline sources are easier than you might think to find online. The Internet Archive full text search, as well as Internet Archive Scholar, are often able to provide a copy or snippet from millions of academic papers, books and even TV programs.
Sometimes, the use of an offline source will be challenged. Be sure to assume good faith for the user who cited the offline source. They might even be able to provide you a scan or an excerpt from that source. Consider visiting your local library to obtain a copy. Even if the library doesn't have that particular book or journal article, it might be available through interlibrary loan. Also consider posting an inquiry on the relevant WikiProject, because some interested editors might have a copy of that source. The volunteers at WikiProject Resource Exchange might be able to help you coordinate your search.