This happens to me as well. Here is my experience and interpretation. Before I turned ~20 years old, I was able to remember almost everything from a book, exce

This happens to me as well. Here is my experience and interpretation. Before I turned ~20 years old, I was able to remember almost everything from a book, except exact numbers and foreign names. Along with getting older, I could remember less and less. However, I feel a good book has greater and greater influences on me. It makes me think more, gives me more perspective, makes me realize more how little I know and understand. I paused more often to think what I just read, instead of just browsing through the book. This also applies to technical books. Take books on programming languages as example. When i was young, I could remember most of its syntax after finishing a book on a new language, and I could jump into writing a program using the language that could actually be compiled and run. Now, I cannot remember much about syntax, but I feel I understand more about the new language, in term of its design, internals, pros and cons, etc. When I am ready to get hands dirty, I have to keep checking manuals for syntax.