"He copies the notes into the notebook."
Translation:Li kopias la notojn en la kajeron.
Kajero is only one word - a word by itself. Could etymologically come from French "cahier" (same meaning, same pronunciation as "kaje").
Could "notlibro" be acceptable? Maybe - however a "notebook" is not really a book, even if that is what happened by usage in English, which of course is not the only language besides Esperanto.