Half wave rectifier input voltage and current waveforms
As @Edgar Brown stated, the two figures are not related.  In the book, the half-wave rectifier shows the voltage and current waveforms with an ideal diode in the first figure.  The second figure assumes that there is a theoretical input voltage Vs and a theoretical input current Is and is used to demonstrate the calculation of the displacement angle.  Since Is is a square wave, the displacement angle is calculated from the fundamental frequency component of Is labeled Is1.  It is only shown to support the concept of the displacement angle calculation.  I can see that the figure's placement in the text without a clear reference to the displacement angle calculation  could be a little confusing.