The Beast Under the Wizard's Bridge by Brad Strickland was the final book in the John Bellairs's series with a dust jacket and frontis illustration by Edward Gorey. Published by Dial Books for Young Readers in 2000, this was probably the last illustration project completed by Mr. Gorey before his death in April of that year.
The preliminary art for the "beast" was still on his drawing table when Kevin McDermott photographed Mr. Gorey's home just days after he died. It can be seen in Mr. McDermott's book Elephant House, The Home of Edward Gorey (see my posting from May 29, 2011 for information on this book).
The dust jacket painting and the black and white frontis illustration were included in the 2001 Bromer Booksellers catalog. I really like the frontis art for this title, with the stained glass window in the stairwell.