Why would "I have the final books" not also be correct?
Feel free to report.
Is "I have the latest books" also correct?
According to http://www.mijnwoordenboek.nl/vertaal/NL/DE/laatste I'd guess yes.
last and latest have a different meaning in English. last is general while latest means the most recent
Can boek also be booking an appointment or something?
i guess there is no more books
r.i.p books 2020
What is the singular form of laatste?
Adjective declension depends on more than just a singular or plural noun, for more info: https://www.duolingo.com/comment/3888221
Ik heb het laatste boek
Singular is equal in this case as boek is a "het"-word (neuter)
Laatste means the last edition books - new?