This book has largely been superseded by Anthony Birley's African Emperor in the late-1980s. Platnauer published this around World War 1 and it is very useful as being the first modern monographic biography, largely sourced on Herodian and Cassius Dio with some of the Historia Augusta used as source, but generally questioned as that book was becoming regarded as less than credible in Platnauer's day.
Platnauer covers Severus background, his rise up the Cursus Honorum and rise to power in the coup d'etat of 193.
Platnauer covers the civil wars and some of the leaders of the factions. he goes on to describe the invasions of the Parthian Empire as well as Severus's visit to Egypt.
Severus then went to Rome where intrigue occurred between Severus, his cousin Plautianus and his sons Antoninus (the future emperor Caracalla) and Geta. These later years seem to get short shrift from Platnauer compared to Birley's later bio.