When you've read a book, can you ever truly forget it?

Started skimming through "Let the Great World Spin," by Colum McCann, in preparation for book group on Monday - not a book that "skims" well, in part because the language is one of its strengths (imagine skimming Ulysses or Mrs. Dalloway? what would be the point?), but it also has a very complex, interlaced plot, which I think it's important that I recall - having read the book several months ago, much of it has slipped from my mind. Memory is a funny thing when it comes to re-reading fiction. If you'd handed me the book and said, here, talk about it, I probably could have given you two sentences: Uh, it's about about the day that a guy crossed between the World Trade towers on a tightrope and about about a few people who saw that event and how it changed their lives. Like a library card-catalogue summary, dull and dumb. But as I skimmed through the first section - which describes the two Irish brothers settled in the Bronx, one of whom, Corrigan (it's their last name, I think, but he goes by that) is in orders and takes up the cause of a group of streetwalkers and falls in love with a single mom/nurse from Guatemala, challenging his faith and vows - and as I read this section the rest of the book just slowly cleared and opened in my mind, like fog lifting or dispersing - it's as if all the other plot elements - the next 300 pages or so - were there all along (they were, of course) but were just inaccessible. I almost don't have to re-read the rest of the book, though I will - but even starting to do so made me think anew about the strangeness of reading and what it means to have read a book, even if we can't recall it. The book is there - it's in us, it has changed us, it's part of us.