Journal of a Residence among the Negroes in the We
Nonfiction, published posthumously. Lewis had inherited a sugarcane plantation in Jamaica.
The Castle Spectre
Perhaps Matthew Lewis's most famous drama. Complete text. Site also offers a bibliography.
The Monk
In frames, at Blackmask Online. The book itself is 824K.
The Monk: A Romance
In plain text, at Project Gutenberg. Also available as a zip file.