John Ruskin's 'The Stones of Venice' and 'Modern Painters' are ambivalent treatments. There are a few others, but they are out of print. And of course, even if the grotesque isn't given extensive formal treatment, its implications can be found in many thinkers. Notable favorites are: Charles Baudelaire, Sigmund Freud, Gaston Bachelard, Antonin Artaud, Walter Benjamin, Georges Bataille, Emil Cioran, Mary Douglas, Susan Sontag, Julia Kristeva. Currently, I'm rereading parts of Leslie Fiedler's classic 'Love and Death in the American Novel' which can be approached as a study of American literature as the grotesque. Limiting ourselves to Western cultures, one can say that the historical avant-gardes especially from Romanticism to Surrealism are impossible without the grotesque, as are Magic Realism, cut-up, and postmodernism, not to mention the Gothic and the Baroque, Mannerism, Northern Renaissance to vanitas, etc. In fact, the grotesque is ever-present throughout the world (pre)history of visual arts, philosophy, music, literature, cinema, folktales, mythology, oral storytelling, propaganda, psychoanalysis, etc...life and existence. Religion is also impossible without the sense of the grotesque, positive or negative. Undertaking an exhaustive survey or even a selective one of the grotesque will be formidable. In truth, many libraries are required for adequate research.
Btw, Quentin Massys' 'Ill-Matched Lovers' (c. 1520/1525) oil on panel painting is at National Gallery of Art, Washington, DC.
When I visited the Palazzo Pitti in Florence, I was happy to see that grotesque decorations pervade its walls and ceilings, belied by its austere stone facade. A great study in contrast.