The sources below differ from a monograph, a book which presents scholarly research on one particular subject.
Encyclopedias and Dictionaries
Specialized encyclopedias offer short (usually 1-2 page) articles on various persons, topics, events, and concepts. Arranged alphabetically, they offer concise introductions to key topics in a discipline, and a bibliography of the most important works for further study. Many are now online; see examples on this page.
Handbooks and Companions
Handbooks and companions are anthologies of articles that summarize the history and current state of research in academic fields and sub-disciplines. The articles are written by top scholars to offer quick introductions to learners, as well as comprehensive bibliographies to aid in further research. Unlike encyclopedias and dictionaries, these works offer longer articles (often 20-25 pages) and are arranged thematically. Find in George with a keyword search such as (companion or handbook) and Hegel.
Scholarly articles are the means by which scholars communicate shorter, more specific questions and answers to one another. Articles are typically 10-30 pages long, and are written (as are books) to contribute to the scholarly discussion of specific questions. They are more focused than books, and the style of writing assumes familiarity with the discipline and particular issues addressed by the piece. These can be found using the library's online research databases, such as Philosopher's Index, Academic Search Premier, and JSTOR (see the Articles tab).
Thanks to Kyle Potter of Georgetown College, Georgetown. Kentucky for these useful descriptions.