For years, Sloan has been keeping a bibliography of everything written about digital reference services at DIGITAL REFERENCE SERVICES BIBLIOGRAPHY: A SUPPLEMENT. This link is to the supplement, and as of today it was last updated on November 18, 2003. Where it is possible, he has provided links to the digital form of the article. If you want to learn about what is happening in digital reference, or are writing a paper, this is an absolute first stop for your literature review of the state of digital or virtual reference.