Calculation Inductance Voltage in book 'Practical Electronics for Inventors'
May be you are right, may be your wrong.

What is pretty sure is that the book is at least ambiguous.

The very first time division on graph - from 0 to 1s- is a half long respect to the other ones.

So if read the graph you are right but if you look at it you can guess that is an even waveform extending simmetrically before 0s mark with the same slope as the other transitions