Dear, your way of asking questions contains some assumptions so these assumptions need to be cleared primarily

First of all you have assumed that knowledge is a thing which can be preserved in mind, and this "knowledge" disappears with the passage of time. Fate of our education system is that we are taught to memorize texts,books, notes etc and according to our poor system, memorization of text is known as "Knowledge" . So here problem with you and other majority of aspirants is that they are always worried that "How to retain that memorized text ". This is unfortunately an unscientific and non academic approach. Mind has its own dynamics of memorizing and retaining information. It all depends on you priorities, your way of thinking, reading and your way of conceptualizing ideas. Arthur Koestler, famous English novelist and writer, once wrote "you can only add to the knowledge, you can not subtract from it"
So there is no need to memorize all the information. Before reading any book ask(and write) questions about that topic and try to answer those questions yourselves.

Your second question is that how many times a book should be revised.Bacon famously said " some books are to be tested, others swallowed, and some to be chewed and digested.
Dear, it all depends that for what purpose you are reading a book.
If you want to memorize complete book than by reading it 100 times your purpose would not be served.If you want to get knowledge than one reading is enough. Through active reading you can not only get information from that book but you can make your own ideas. After a good study, number of questions will haunt you, they will force you to get answers from your surroundings, from your society.Knowledge always increases your thirst for further knowledge.


Your last question shows your level of interest. Your motivation level is too low, in the result you can not focus on your studied. All this happens when you don't have any purpose when you are not dedicated toward your goal.