The most important and inclusive idea has to be the

The most important and inclusive idea has to be the textbook title.

Explanation:

The textbook title is one of the first impressions book buyers receive, so one should be very careful when choosing one. There is some guidance regarding this topic depending on the genre. However, the most important thing one should bare in mind is that the title should be descriptive and indicative of the book's content: the idea conveyed in the title should be comprehensive enough so as to describe the most important elements of the content