To find Manga, use "manga or graphic novels" and "Japan" as keywords and limit to books or use "Comic books, strips, etc. -- Japan -- periodicals" as a subject search. A significant proportion are cataloged under PN6790 J33 or PN6790 J34 and can be found by browsing the shelves in the East Asian Collection or Lilly (for English translations).
For research on manga, search "Comic books, strips, etc. -- Japan -- History and criticism".