In the first figure (a) and (b) show the same circuit except for the diode placement. The text mentions "Suppose the polarity of the diode is reversed as in Figure above (b)" but it's not shown that way in the figure because the capacitor and source are also reversed.
I discovered that when I made the circuits as shown in the figure and didn't get the expected results. Then I remembered the section about rectifiers and realized the issue.
I discovered that when I made the circuits as shown in the figure and didn't get the expected results. Then I remembered the section about rectifiers and realized the issue.