Often when I read a book, I observe little details I will want to be aware of when my children are older (and can read the books themselves) or when recommending the book to others.  They’re minor parts of the book, though – perhaps a vulgarity used or a passing reference to evolution – so I know I won’t remember them months or years later.  What I have begun doing when I come across something like this is to stick a Post-it note inside the front cover of the book, and make whatever notes on it I need to.  This way, I will be able to tell at a glance if a book is something I can, in good conscience, recommend, if it’s appropriate for a child of a particular age, etc.  However, I have not marked the book itself in any way, and can easily remove the note to pass the book on (or if I don’t want the child to see the notes, for whatever reason).