This 1820 print by Shuntai Katsukawa depicts a samurai.
In your book, you emphasize the contrast between ninja and samurai.
Ninja are usually regarded as the anti-samurai. The samurai were extremely overt and colorful personalities. Their whole being depended on public display — death-defying and often death-seeking bravado. The ninja were the opposite: in order to be a spy, you have to survive and be secretive. Secretiveness was something the samurai pretended to despise, but in fact the ninja were vital to military activity. And quite often the samurai during the day doubled as ninja during the night.
So could ninja be samurai at the same time?
You could, theoretically. There would have been some sort of distinction, because samurai were often extremely high class, but ninja not necessarily so. But there was an overlap in the middle.