The word "mook" is a portmanteau of the words "magazine" and "book." I had long assumed, mistakenly, that it was one of those examples of wasei-eigo (Japan-made English terms), such as "open car" (a convertible) or "virgin road" (the aisle down which the bride walks at a wedding ceremony). But from online searches I learned the term apparently originated in Europe, making its first appearance in 1971 at a convention of the London-based International Federation of Periodical Publishers.
Unfortunately, the title and language of history's first mook was not forthcoming, but the genre has become widely popularized in Japan. While it's almost impossible to generalize on the themes, designs and other details of such publications, according to data from the publishing industry, mooks do have certain features in common: They adopt the magazine-code system, which enables them to be treated as magazines for distribution purposes; the size of their print run typically exceeds those of books, enabling them to achieve greater economies of scale; unlike books, they may run advertisements, helping their publisher recover initial costs; and likewise, many are believed to be put together at comparatively lower costs by outsourcing editing and production work.
Mooks also give publishers more leeway than magazines in terms of setting sales dates, the size of the print run and sales price. And, in general, they do not carry a sales expiration date (a deadline for unsold returns), enabling them to remain on sale for extended periods.