They're the worries expressed by James Collins in an essay in the New York Times, and they boil down to the fact that he doesn't remember the books he reads. This applies even to books he loves: 'all I associate with them is an atmosphere and a stray image or two, like memories of trips I took as a child'. The question that follows: Why read books if we can't remember what's in them? OK, for pleasure, he allows, might be one answer; but don't we also want to learn something? In due course, Collins is cheered a bit by a professor of child development, who reassures him that he's a different person for reading the books he does, because 'reading creates pathways in the brain', the information gets stored in networks, and suchlike.
I have three thoughts about this. First, reading for pleasure, even if that's your only purpose in reading, is just fine. You play poker and you enjoy it; you go for a walk, watch a sunset, and enjoy it; you read and you enjoy it. Where's the problem? But, second, while I too forget much of the content of the books I read - it seems inevitable unless one just re-reads the same books all the time or has a phenomenal memory - it is not generally true that the whole book is 'lost'. There may be memory of the overall shape of the plot or the thesis, of a character, theme, or incident or argument. If one reads several books by the same author, there's a sense of the mind of that person, of his or her central preoccupations, and so forth. Third, if you care about remembering a book, there's a simple solution: take thorough, detailed notes on it. You can sometimes then retain the contents for a lifetime.