Complex sentence: They started as single volumes with numbers (BB nn), then as double novels with numbers (BDN nn), and finally as Doc Savage Omnibuses (DSO) with three, four, or five stories without numbers.
Split into simpler sentences: They started as single volumes with numbers (BB nn). As the stories got shorter, Bantam combined double novels with numbers (BDN nn), and finally Doc Savage Omnibuses (DSO nn) with three, four, or five stories without numbers.