If a quotation already contains bracketed text, avoid adding your own bracketed text, since multiple sets of brackets may be confusing for the reader. However, if you are unable to rephrase the sentence to avoid adding bracketed text of your own, include a note in parentheses explaining which brackets are present in the source.

Allen writes that “Wilson’s progress [on the new novel] was substantial that year, but Wilson was still ‘concerned that the book might not be completed by the [publisher’s] final deadline'” (Minnesotan Novelists 200; second set of brackets in source).