Final answer:
The reading level of a nonfiction book is the primary factor in determining its appropriateness for an intended audience. Adjustments in word choice and style are essential to align with the readers' understanding and interests.
Step-by-step explanation:
The factor that determines whether a nonfiction book is written appropriately for the intended audience is primarily the reading level of the book. When considering the creation of creative nonfiction, one of the essential elements to keep in mind is the audience. The content, word choice, tone, and overall style must align with the reading ability of the intended readers. If the audience can't comfortably understand the language and concepts presented, the book will not serve its purpose effectively, whether it's to inform, persuade, or entertain.
In addition to reading level, the writing process should take into account the audience when considering factors like intent and reliability. For example, the vocabularies and concepts used in a book intended for professionals in a field will differ from those in a book intended for a general audience. Likewise, a book for children will use simpler language and potentially more illustrations than a book for adults.
The other factors mentioned, including the length of the book, the font size, and the cover design, whilst influential in the overall presentation, do not fundamentally determine the appropriateness of the text for its audience in the same way that the reading level does.