Oxford Art Online
The go-to resource to find basic reference-type information about an artist. Use it as an alternative to Wikipedia – which can be a perfectly fine source for some things but in general tends to be less accurate, reliable, and complete than a scholarly published source – to learn about the career and works of the artist in question. A good reference source such as OAO can function as a supplement to monographic books, or books that cover the career and works of a single artist, in providing overview and background information that gives you a context, or a contour, within which to place what you find in the more in-depth sources.
IMPORTANT: at the end of the biographical entries you will find bibliographies of scholarly works considered standard. Consulting these bibliographies is a great way to get an overview of important scholarship and to find the best monographic works (this can be useful because, while there may be several monographs on a given artist, they are not all equal in terms of quality).