I'm not sure about the tablet analogy, but regardless this is great advice. It's not enough to just read something. It helps tremendously to be engaged while reading -- to take notes, and discuss ideas with your friends. Then, after you've finished, come back in a few days, and again in a few weeks to reflect on things. The amount of text and information contained in books (and on blogs, on HN, in emails, etc) is overwhelming, and it's asking too much to try and remember it without reflection or repetition.
This reminds me of something my Latin teacher, who was an ancient history buff, used to always claim -- that many Roman citizens could go listen to a speech for an hour, then recite almost the whole thing from memory. Enviable to be sure, but remember that the vast majority was illiterate, and even for those who weren't, chiseling letters into stone tablets was no easy task.
That's why Plato (or Socrates or so? One of the Greek philosophers) ranted against writing --- it makes people scatterbrained by allowing them to be scatterbrained, instead of having to remember everything.
This reminds me of something my Latin teacher, who was an ancient history buff, used to always claim -- that many Roman citizens could go listen to a speech for an hour, then recite almost the whole thing from memory. Enviable to be sure, but remember that the vast majority was illiterate, and even for those who weren't, chiseling letters into stone tablets was no easy task.