Primary sources in literature are original, uninterpreted information (often, but not exclusively textual) relevant to a literary research topic. Examples include original works of fiction, art, or music; letters; diaries; interviews; or even works of criticism.
The key question to ask when trying to classify a source as primary versus secondary is how you intend to use it. If a work was written or created during the time period that you are researching, it can be used as a primary source.
For example:
A 1922 review of T. S. Eliot's The Waste Land would be considered a secondary source if your project is an analysis of Eliot's poem, but would be a primary source if your topic is the critical reception of Eliot's works, or the perception of modernism as a literary style in the 1920s.
Visit Special Collections to use original texts in print or manuscript.
In addition to the digital resources listed in this guide under the tabs above, here are tips for finding printed/published 
versions of primary sources.
Detail from early manuscript of The Sun Also Rises.
Image via avrenim_acceber at Flickr, used under Creative Commons license.