International Medieval Bibliography (IMB)
A comprehensive, current bibliography of journal articles, essay collections and conference proceedings for all areas of medieval studies.
Mirabile (esp. Medioevo Latino or MEL), -- Searches across several databases of medieval studies, including indexes of articles and books, with about 1,400 full-text articles.
Scholarly journal articles, books, and dissertations on world history; a good supplement to the IMB, especially for research on the historical context of the Waltharius.
MLA International Bibliography
The standard North American bibliography for scholarly literature -- articles, essays, dissertations, and conference papers -- on literature, language, and folklore.
"Attila and the Huns" -- annotated bibliography in Oxford Bibliographies Online
"Waltharius." Seven-page entry by Dennis M. Kratz in German Writers and Works of the Early Middle Ages: 800-1170 (print only; Dictionary of Literary Biography, vol. 148)
For many more sources like these, search in the HOLLIS Advanced Search for the Subject Middle Ages encyclopedias.