sharepoint enterprise – API Call to Document Library, how to get a subfolder’s name?

I have been trying to make a call to a specific subfolder. Oddly if I call directly to:

'sites/website/RootFolder/Folders('subfoldername')/Files

It returns the subfolder items but it won’t give me additional columns like Category or Author.

I can get all the data by doing a regularly api call but it gives me all folders. I need to filter it to one folder but I am not sure what column type to select for this?

The call I am doing but don’t see the “folder” names?

    var sel='FileLeafRef,File_x0020_Type,Category,Category0,OData__Author,Folder,File,ContentType';
    var url=pth+'/_api/Web/Lists/GetByTitle('Library')/items?$select='+sel;