This article is one of the good article of QT for S60.This article gives all the information about system using QSysInfo class.
XQSysInfo API is very useful for QT.This have similar operation like Qsysinfo.This class gives all information about system.XQSysInfo gives all information about SIM like Network name and Operator name or battery level.
The main difference is there this class dose not use any API which require certified signing.