This is a difficult book cover to design.
Emacs is a text editor; computer programmers use it to write code, write e-mails, write documentation and administrate servers.
The official mascot for Emacs is the animal the "gnu" (also known as a "wildebeest")
I would like a book that involves the gnu/wildebeest. You're free to choose how stylized it is; whether it's very realistic or more on the cartoony side. No humorous children's book cartoon character though.
Other aspects worth considering is that Emacs is often (humorously) referred to as the "Church of Emacs". But if you do decide to go down the route please make sure you make no references to specific religions. You can come up with humorous religious iconography BUT NO SPECIFIC RELIGIONS.
Another in-joke in the Emacs community is that it is called "the kitchen sink editor" (because it can do everything). You're free to play with this concept also.
I personally like the art deco movement (Google Images search for "art deco movement"). If you can make the book look like that then that would be great.
The book title is "Mastering Emacs" and the author is "Mickey Petersen".
The book is typeset in Monotype's Bembo. Please pick a font -- serif or sans serif -- that works well with this font.
You are otherwise free to experiment and come up with interesting designs.