Art History and Visual Arts

The best modern art encyclopedia, searchable by concepts as well as by artists' names. The bibliographies are a worthwhile starting point for further research

Regularly updated annotated bibliography addressing themes and topics in Architecture.

Developed cooperatively with scholars and librarians worldwide, Oxford Bibliographies offers authoritative research guides. Combining the best features of an annotated bibliography and a high-level encyclopedia, this resource guides researchers to the best available scholarship.
From their website

Regularly updated annotated bibliography addressing themes and topics in Renaissance and Reformation Studies

From Duke University, a guide to the publications, methodologies and backgrounds of leading art historians.

The Grove Encyclopedia of Medieval Art and Architecture offers coverage of all aspects of art and architecture from Medieval Western Europe, from the 6th century to the early 16th century. The Encyclopedia draws upon the scholarship in the Grove Dictionary of Art and adds hundreds of new entries on topics not previously covered, as well as fully updated and expanded entries and bibliographies.
Adapted from their website.

Full-text version of Dictionary of Architecture, Concise Oxford Dictionary of Art and Artists, Concise Oxford Dictionary of Art Terms, Dictionary of Twentieth-Century Art and more. Provides access to over 120 language and subject dictionaries, reference works, and biographical directories in a variety of disciplines published by OUP that can be searched individually or in one cross-searchable database.