Biographies give information about a person's life. For your purpose, the most useful type of biographies are critical biographies, which focus on an author's work and can help you better understand how it fits into an author's life. Perhaps even more usefully, they can help you understand the historical context of a work, and often include information about its reception when it was published.
Author interviews are an important resource, especially for contemporary authors. These are also useful for understanding context and for making connections between the work in question and ideas expressed in it.
Interviews are very important resources when you are interested in contemporary authors.
In interviews, authors will often:
You may be able to use this information as further evidence that certain events or ideas are important. Interviews may also provide context for the work.
Finally, for contemporary works, there may not be much criticism yet, so interviews are often your best resource.
It depends on what kind of interview you need!
Newspapers, popular magazines, and radio shows often publish interviews. However, these are often brief and not as interesting as the longer interviews you will find in scholarly journals or similar publications. Brief interviews of this sort are likely to turn up in a basic web search, but they are also included in resources such as:
Literary experts conduct longer, more complex interviews. These can also sometimes be found on the web, I can recommend the following academic resources:
Video interviews are especially interesting because they include the author's voice and body language. You might find them in many places, including:
Many author interviews are also posted online! If you are interested in a living author, I'd recommend checking whether they have included links to any of their articles on their website; they often have "Press" sections for exactly that purpose.
Aside from just doing a web search, I can also recommend a few sites:
Why might you consult a biography?
Getting the facts right. It's possible you just want to reference a few facts about the author. A short biography is best for this purpose.
Reception studies.Biographies will often provide information about the reception of author's works.
Historical Context.If you are interested in positioning a work historically, a biography can be a good place to start. Here, longer or book-length biographies are likely to be useful.
Biographical Analysis. Maybe your primary interest is analyzing a work in view of the author's life. While there are many other ways to approach a literary work, this can sometimes be fruitful. It's probably a good idea to consult many different biographies, including book-length ones, if this is the case.
Short biographies are great if you just want to get some background information on an author.