occasionally I'll pick up a book and start reading and at some point come across a sentence or a paragraph that seems familiar and realise that I've already read this book but entirely forgotten that I had. Then I'll have an exciting voyage of rediscovery because it's not true that you can't discover a good book twice.
And I don't mind forgetting bits of a really
good book because when/if I revisit it I can recreate something of the experience of first reading it, which if I'm re-reading it must mean it was really good. Unless it was really
bad and I've just forgotten how bad it was!
I think I tend to retain the basic plots even if some/most of the details fade over time. I know I read Poe's 'The Gold Bug' back in the mid-80s and it was good though I don't recall too much of the narrative but it was a quarter of a century ago so I guess that's only to be expected. With a novel like The Lord of the Rings
, which I read 8-10 years ago I probably remember most of the plot, enough to know roughly where to find a particular passage if I want to but I probably knew the book better five years ago when it was fresher in my mind.