Based on concepts popularized by Joyce Saricks (2005) and Nancy Pearl, appeal is a way of looking at reading. Instead of focusing on the genre of a book, appeal isolates the reading experience that a reader is looking for and enjoys. Rather than looking at what the book is about, these experiential elements of appeal let us identify what about the book draws the reader in.
Below is an outline of the four appeal elements that are used to understand fiction and narrative non-fiction. I've also reflected a bit on how they play out with regard to the non-narrative non-fiction that I read for pleasure.
Story
If a reader likes books heavy in the story appeal factor, they want a page-turner. They want to be sucked into a fast-paced narrative that makes them lose track of time until they arrive, breathless, at the end. These stories are highly plot driven.
Carried into the non-narrative non-fiction that I tend to read, story tends to manifest for me as the author climbing toward a "slam-dunk" argument. I want to know more about the topic and I get caught up in following the author's constructed argument toward its climax.
Character
Books that are strong in the character appeal factor are ones where the characters seem real and alive. The reader identifies with the people they're reading about, and feel like they're losing a friend when the story is over.
For non-narrative non-fiction, I find that works strong in character are about a specific person or people, as opposed to a more general idea or concept, and that person or people are integral to understanding the work. Character can also come into play in these works if I strongly identify with the subject - in a sense, I reflect into the work and become a type of character in the work.
Setting
Setting-strong books are rooted in a particular place or time - it feels like the reader is really there. A book that has a strong setting appeal cannot be transposed into a different place or time without the nature of the book irrevocably changing.
For non-narrative non-fiction, this manifests as the work's subject and focus being strongly about a particular place or time, as opposed to a person or peoples, or a specific argument.
Language
Books that have a high language appeal are all about the way the author writes and constructs sentences. A reader who loves language-driven books will read slower and slower in order to enjoy the craftsmanship of each paragraph.
The language appeal factor plays out very specifically for me in non-narrative non-fiction. I do not like language-heavy books for fiction and narrative non-fiction. However, I enjoy certain non-narrative non-fiction works that are written in a strongly "academic" style of writing (as I will discuss below). This is a very personal manifestation of this appeal factor, so this may not be a universal experience.
References
Saricks, J. (2005). Readers' Advisory Service in the Public Library, 3rd ed. Chicago: American Library Association.