Did you get the AOR by email ? If so, you won't be getting the book by regular mail. Your AOR email as well as ECAS should have a link where you can download the pdf or audio version, and order a physical copy free of cost, should you choose to. IRCC no longer sends a physical copy, unless you do not provide an email address on the application form, in which case they will send your AOR and Discover Canada by regular mail.
Copying the link here for your convenience:
https://www.canada.ca/en/immigratio...cover-canada/read-online/study-questions.html
Edit: also would like to mention again that since your parents are > 55 years old, they are exempt from the test. Assuming they are being processed under bill C6 regulation.