Complex sentence: Radcliffe's novels, above all "The Mysteries of Udolpho" (1794), were best-sellers, although along with all novels they were looked down upon by well-educated people as sensationalist entertainment.
Split into simpler sentences: Radcliffe's novels, above all "The Mysteries of Udolpho" (1794), were best-sellers. However, along with most novels at the time, they were looked down upon by well-educated people as sensationalist entertainment.