A zyBook code uniquely identifies a particular zyBook, typically for an instructor's specific class in a given term at a school. Instructors can configure zyBook content, so every class' zyBook may be unique.
A zyBook code looks like: SpringfieldUnivCS101HomerSimpsonSpring2014 (just an example).
Instructors usually provide students with a zyBook code in a syllabus, announcement, or web page.
After a student creates an account and signs in at zybooks.com, the student is taken to their library, where a box appears to enter a zyBook code:
Entering the code takes the student to their zyBook's home page, where the student can subscribe. Once subscribed, then in the future the zyBook will appear in the student's library.
If your zyBook code is unrecognized, check the spelling (try copy-pasting). If that fails, contact support@zybooks.com with info like the zyBook code, instructor's name, school, class. Also let your instructor know, so other students don't have the same issue.
NOTE: Some schools use "single sign on" in their learning management system (LMS) like Canvas, Blackboard, Moodle, Sakai, D2L, etc. For those schools, instead of creating an account at zybooks.com and entering a zyBook code as above, students should instead just click on the zyBook link in their LMS. That creates an account using the student's school email, and goes to the class' zyBook home page, where students can then subscribe.