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. In fact, many great sources are only available offline. Don't let that fact scare you away from using them as a source in Wikipedia.
Books are a typical example of an offline source. These are often great resources, with information that can't be found online. Several ongoing projects, such as Project Gutenberg and Google Book Search, aim at digitizing certain books 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).
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.
Special care should be taken when using offline sources. Make sure to provide full bibliographic information, often by using a fully-filled out citation template, like {{cite book}} or {{cite news}}. Complete information helps Wikipedia's readers find the source when they need it, and also increases the source's credibility among the Wikipedia editing community (who may otherwise be skeptical of its reliability). Second, use the quote= parameter within those citation templates to provide 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.
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.
Articles must be written • Be a reliable source • Cohesion • Concede lost arguments • 8 simple rules for editing our encyclopedia • Explanationism • Here to build an encyclopedia • Paradoxes • Product, process, policy • Purpose • Ten Simple Rules for Editing Wikipedia • Trifecta • User Creed • Wikipedia in brief • Wikipedia is an encyclopedia • Avoid vague introductions
Alternative outlets • Articles with a single source • Avoid template creep • Bare notability • Bombardment • But it's true! • Citation overkill • Clones • Coatrack • Discriminate vs indiscriminate information • Existence ≠ Notability • Explanationism • Google searches and numbers • High Schools • Inclusion is not an indicator of notability • Independent sources • Inherent notability • Insignificant • Masking the lack of notability • Make stubs • No amount of editing can overcome a lack of notability • No big loss • No one cares about your garage band • No one really cares • Notability/Historical/Arguments • Notability is not a matter of opinion • Notability means impact • Obscurity ≠ Lack of notability • Offline sources • Notability sub-pages • One sentence does not an article make • Other stuff exists • Perennial websites • Pokémon test • Run-of-the-mill • Subjective importance • Third-party sources • Video links • What notability is not • Wikipedia is not here to tell the world about your noble cause
Construction
100K featured articles • A navbox on every page • Alternatives to the "Expand" template • Amnesia test • An unfinished house is a real problem • Avoid mission statements • Bare URLs • Beef up that first revision • Concept cloud • Don't demolish the house while it's still being built • Don't hope the house will build itself • Don't leave giant breaks between sections • Don't panic • Editing on iPhones, iPads, etc. • Editors are not mindreaders • Featured articles may have problems • Give an article a chance • Inaccuracies in Wikipedia namespace • Link rot • Not everything needs a navbox • Not everything needs a WikiProject • Nothing is in stone • Over explained • Permastub • Potential, not just current state • Put a little effort into it • Pruning article revisions • Restoring part of a reverted edit • Robotic editing • Temporary versions of articles • The world will not end tomorrow • There is no deadline • Wikipedia is a volunteer service • Wikipedia is a work in progress • Wikipedia is not being written in an organized fashion • Write the article first • Writing better articles
Deletion
Adjectives in your recommendations • AfD is not a war zone • Arguments to avoid in deletion discussions • Arguments to avoid in deletion reviews • Arguments to avoid in image deletion discussions • Arguments to make in deletion discussions • Avoid repeated arguments • Before commenting in a deletion discussion • But there must be sources! • Content removal • Delete the junk • Does deletion help • Don't overuse shortcuts to policy and guidelines to win your argument • Follow the leader • How to save an article proposed for deletion • I just don't like it • Immunity • Liar Liar Pants on Fire • Nothing • Overzealous deletion • Relisting can be abusive • Why was my page deleted?
About essays
Value of essays • Difference between policies, guidelines and essays • Essays are not policy • Don't cite essays as if they were policy • Finding an essay • Quote your own essay