Translation:They call you.
The suffix ... -mu = your (bukumu = your book). -mu is short for kamu. Also, bukuku (-ku is short for aku); and -nya (bukunya) is short for his/her or 'the'. The last one depends upon the context. You can only use -nya for his/her if you have already referred to him or her earlier in the sentence or conversation.
-ku and -mu may be used at any time.