I recently read, and really enjoyed, Death Comes to Pemberley, which is a murder-mystery sequel to Pride and Prejudice, written by PD James.
Wide Sargasso Sea is famously a prequel to Jane Eyre.
But are there any other good ones like these? I'm less interested in a "retelling" — more a sequel or prequel, written by a new author, to an existing work.
by extraneous_parsnip