Memoir and Memoirs both can be correct depending upon the context.
If you are referring to a single written work, you may use the word 'Memoir'. For example, "I'm reading the memoir, 'The Glass Castle'".
However, if you are referring to more than one written work, then you will use 'Memoirs'. For example, "Rosa Parks had many memoirs".
It is also worth noting that memoir is a type of biography, typically written by the person who is the subject of the work, while biography is typically written by a third-party.
For a more detailed explanation and comparison between memoirs and biographies, you may refer to this article:
https://www.diffen.com/difference/Memoir_vs_Biography
