We cannot know what your textbook gives as the main reasons behind the internment of 1942. However, there are three main reasons that are typically given.
First, there was fear. In 1942 when the executive order was issued, the US had undergone a major surprise attack at Pearl Harbor only a little more than two months before. Days afterward, a Japanese submarine shelled California. The US was losing the war in the Philippines and elsewhere. Things did not look good. Therefore, it was quite unsurprising that the US would have been fearful and ready to take drastic actions to prevent further attacks.
Second, there was a lack of time and resources. In the context of the war, it would have been exceedingly difficult to take the manpower and the time to investigate the 100,000 or so people of Japanese descent on the West Coast.
Finally, there was racism. Americans had long held racist views of the Japanese and other Asians. Therefore, they were willing to suspect people of Japanese descent of great treachery. They were also willing to act on their suspicions because they did not care much for the people of Japanese descent.
It was mainly for these reasons that the internment was ordered and carried out.