[jhonny]

Supongo que Book:NO es un parametro de salida, entonces la cosa deberia ser, asi:

Código SQL [-]
begin

  Select BOJ.BOOKINGNO
  from BOOKINGS_OBJECTS BOJ
  left join BOOKINGS BO
    on BO.BOOKINGNO = BOJ.BOOKINGNO
  where
    BOJ.BUILDINGNO = :"BUILDING_NO" and
    BOJ.OBJECTNO = :"OBJECT_NO" and 
    BOJ.DATETO <= :"DATE_SP" and
    BO.STATUS = 3
    Into :BOOK_NO; /*Ha Book_NO le faltan esos dos puntos, para indicarle que es un parametro de salida*/

  if (not :BOOK_NO is Null) then /*Quitale esas comillas, me temo que el motor dira que nunca es Null ya que siempre es igual a "BOOK_NO"*/
    SUSPEND;
  else begin

    Select BOJ2.BOOKINGNO 
    from BOOKINGS_OBJECTS BOJ2
    left join BOOKINGS BO2
      on BO2.BOOKINGNO = BOJ2.BOOKINGNO
    where
      BOJ2.BUILDINGNO = :"BUILDING_NO" and
      BOJ2.OBJECTNO = :"OBJECT_NO" and
      BOJ2.DATEFROM <= :"DATE_SP" and
      BOJ2.DATETO >= :"DATE_SP" and
      BO2.STATUS <= 2
      Into :BOOK_NO; /*Lo mismo aca*/
    SUSPEND;

  end

end>