The standalone/series tension, how to resolve it
Every series first book must pass two tests: it must feel complete when read alone, and it must make readers want more. These pull in opposite directions. The resolution is to distinguish between story arcs. The book-level arc, the central conflict and emotional journey of this book, resolves. The series-level arc, the overarching question that runs through all the books, does not. A reader who finishes book one should feel satisfied and unsettled at the same time. Satisfied by what was resolved. Unsettled by what was not.