Aimed at younger readers, these books were published only in
paperback form, with brightly coloured covers and were heavily illustrated in black and white. Unlike other interactive novel series such as
Choose Your Own Adventure stories or
Fighting Fantasy gamebooks, each Interplanetary Spy book is comprised largely of illustrations in a style that mixes
comic book-like line drawings with blocky, straight edge illustrations matching the graphical quality of
video games from the time of publication. Rather than requiring readers to select from various actions and directions, or succeed in dice-driven 'combat' to progress through the story, these novels mostly involve puzzle solving. Image-based challenges such as mazes, pattern matching, and visualization problems are common throughout the series, with all stories being presented in
second-person point of view. The writers frequently used the books' own
ISBN numbers, which they called "Interplanetary Spy Binary Numbers," as codes or clues within the stories. By including blank spaces or boxes for writing, puzzles that required pages to be folded over and even a cutout model in one novel (
The Star Crystal) the reader was actively encouraged to make changes to the books themselves. Because of this, it is almost impossible to find a copy today in
mint condition. ==Book Series==