Student or Learner
in "teacher's book" 's is used but it isn't used in "student book"
or other examples like "john's book" "cigarette ash"
why sometimes 's is used and sometimes it isn't ?
Sometimes it is a possessive [ teacher's book = the book belonging to the teacher; John's book - the book belonging to John ]; sometimes is is an adjective [cigarette ash = ash from a cigarette; student book = book for use by students].