As I look at implementing GetCellFormula(int index) for XLS, there are some dependencies, for example defined names. These are defined on the worksheet and workbook level. See, for example https://docs.devexpress.com/OfficeFileAPI/DevExpress.Spreadsheet.DefinedName.

To help shape how I implement this, would it be useful to expose a class like

public class DefinedName
{
    public string Name { get; }
    public string Formula { get; }
    public bool IsGlobal { get; }
}

and an API like

public interface IExcelDataReader
{
    public IReadOnlyList<DefinedName> GetDefinedNames();
    public IReadOnlyList<DefinedName> GetDefinedNames(int sheetIndex);
}

What do you think?