Complex sentence: The Library of Congress online catalog lists volumes 11 to 13 as forthcoming September 2014--which will complete the series: according to illustrator Gurney, 13 titles are planned.
Split into simpler sentences: The Library of Congress online catalog lists volumes 11 to 13 as forthcoming September 2014--which will complete the series, Gurney implies. According to illustrator Gurney, 13 titles are planned.