As a BSS engineer, you have two main field: radio and equipments. first you have to know on which side you are.
if Radio : buy a typical telecom book and study fundamentals of GSM. Radio is about the same in any constructor.
if equipment : you need to get documentations about the equipment you’re supposed to take care of. It is dependant on the vendor (ericsson ? nokia ? alcatel ? motorola ?) and depndant on the system (BTS ? BSC ? TC ? Abis ?)
If you’re new and someone hired you to do these tasks, you need trainings. Ask your employer to purchase trainings from your telecom vendor…