I kinda have two modes of reading - my "recreational" mode where I read faster and forget faster, so I can re-read the book a year later with little less enjoyment than the first time; and my slightly slower but more permanent "study" mode where I can often give a coherent summary a couple of years later and remember significant passages (which is helpful for pedantic geeks like me). Unfortunately after 10 years I've generally forgotten most of it, whichever way I read it, so I've taken to re-reading a few things every 10 years. eBooks are great for storing and carrying around in that sense