Hi!
Welcome!
As for your questions, to add a book manually, go to menu Book- Add. There appears a book card with empty fields. So, fill the in, add the cover and all the other info about the book.
To number the books within a series, you need to use the field Volume in Additional tab in the book card. Unfortunately, in the current version this field cannot be displayed in a separate column on the left, but, happily, this option will be added already in the next version.
Regards,