If the book contains no ideas next month I hardly can remember about what it was.
If author of the book actually bothered to put some ideas in then I remember those ideas and usually the book in which I encountered them for many years. vide Iain M. Banks "Use of weapons", Orson Scott Cars "Ender's Game", Vernor Vinge "Fire upon the deep" or Henry Kuttner novels, Terry Pratchett "Nation"
If you don't remember the books you've read then change the books you are reading to some that have actual content not just crafty words.
If author of the book actually bothered to put some ideas in then I remember those ideas and usually the book in which I encountered them for many years. vide Iain M. Banks "Use of weapons", Orson Scott Cars "Ender's Game", Vernor Vinge "Fire upon the deep" or Henry Kuttner novels, Terry Pratchett "Nation"
If you don't remember the books you've read then change the books you are reading to some that have actual content not just crafty words.