Digital modulation techniques are seeing increasing use worldwide. In the US, the FCC is actually mandating narrowband digital mobile radio (DMR) for certain Land Mobile Radio (LMR) users as explained in this Digital Mobile Radio Association white paper. Datasheets for various chips increasingly made reference to acronyms like GMSK, FSK and others.
Agilent has published this comprehensive 48 page application note, Digital Modulation in Communications Systems – An Introduction, which provides a clear, authoritative explanation of digital modulation. Definitely worth the read!