Descartes lived in a time when most people couldn't even read and therefore the people who actually wrote books might well have been some of "the greatest minds who ever lived". Today almost everyone can read and almost everyone has the potential to write a book. So if we are talking about baseline quality, the magic of books has decreased a lot since the enlightenment. Today a book is just an indication of quantity of writing and not necessarily an indication of quality. And it isn't like the "great minds" aren't also on Twitter or writing shorter form articles. The quality is there too, it just might take a little extra effort to find it.