How read or write in the summary information of an Offiche document ?

An Office document file is a structured storage file that an application can read with the StgOpenStorage function from the Windows API. This kind of file is made of storages and streams.
COM defines a standard common property set for storing summary information about document. This information is stored in a stream under the root storage. The following function shows how you can get the author property by giving a filename :

uses ActiveX, ComObj, SysUtils;

function GetSummaryInfAuthor(FileName: TFileName): string;
var
  PFileName: PWideChar;
  Storage: IStorage;
  PropSetStg: IPropertySetStorage;
  PropStg: IPropertyStorage;
  ps: PROPSPEC;
  pv: PROPVARIANT;
const
  FMTID_SummaryInformation: TGUID = '{F29F85E0-4FF9-1068-AB91-08002B27B3D9}';
begin
  PFileName := StringToOleStr(FileName);
  try
    // Open compound storage
    OleCheck(StgOpenStorage(PFileName, nil, STGM_DIRECT or STGM_READ or STGM_SHARE_EXCLUSIVE, nil, 0, Storage));
  finally
    SysFreeString(PFileName);
  end;

  // Summary information is in a stream under the root storage
  PropSetStg := Storage as IPropertySetStorage;
  // Get the IPropertyStorage
  OleCheck(PropSetStg.Open(FMTID_SummaryInformation, STGM_DIRECT or STGM_READ or STGM_SHARE_EXCLUSIVE, PropStg));

  // We want the author property value
  ps.ulKind := PRSPEC_PROPID;
  ps.propid := PIDSI_AUTHOR;

  // Read this property
  PropStg.ReadMultiple(1, @ps, @pv);

  Result := pv.pszVal;
end;


See http://msdn.microsoft.com/library/default.asp?url=/library/en-us/com/stgasstg_7agk.asp for more information about the Summary Information Property Set.