I've flipped through literature anthologies and literary histories today looking for annotations and I must say that the Norton anthologies are very time consuming because their pages are so thin. The Oxford anthologies are much easier to deal with.
Probably of more interest is how Fr. Ong's treatment of books. What I mean, actually, is that he seems to have had a manuscript culture like attitude towards books that goes well beyond normal academic glossing of texts. While many books have few or no annotations at all, others are extensively marked up. Fr. Ong will add his own categories in indexes as well as supplement existing index entries. And occasionally he'll add in pages with comments and ideas. The most extensively modified book I've come across so far is a comb-bound Outline-History of English Literature, which, admittedly, is more open to such modification than say a monograph on Milton. Inserted throughout the two volume work are additional pages of notes, quotes and paraphrases from articles and the like typed and given page numbers like 89a-b.