Whenever I peruse my bookshelves looking for something, I find books I forgot I had. So they're not only doing me no good (sitting there collecting dust), I don't even remember that I HAVE a lot of 'em, which reduces their chance of being re-read to zero. To me, this translates into, "This can GO" -- and I usually foist 'em off on friends who throw/give NOTHING away, so that they'll be THERE if I DO need to re-read 'em. (Did any of that make sense?)

Anyway, that's my half-assed system.