The book is a short history of the computer industry, starting with relay computers and ending with the IBM PC, introduced in 1981. Before digital computers, there were digital switching systems, used by the phone companies, made with relays. With the earliest digital computers, users found relays to be just too slow, and vacuum tubes were mainly used in analog devices, like radios. But soon digital counters were invented, showing that vacuum tubes had digital uses also. But vacuum tubes had their own problems, producing too much heat and failing at random. Vacuum tube computers also used too...