This is entirely up to you – it’s your book! However, you should abide by industry standards for the most professional look. For example, JK Rowling would not need to include “Book 1 of the Harry Potter Series” as a subtitle, since that information will appear under the series info in the metadata.
Additionally, your subtitle should match the information which appears on the cover. If your cover says: “Fahrenheit 451: A Novel,” your metadata should not say “Fahrenheit 451: A Memoir.”