Think of information as puzzle pieces. Managers tend to give their assistant a few pieces of the puzzle here and there. Maybe you do this with most of your staff members. Remember, though, this relationship is supposed to be different.
Your assistant is supposed to be your right arm; your liaison – speaking on your behalf; directing the flow of information on your behalf and more. For that person to be able to do a top notch job, they need certain information. Your assistant may need clarification, or details, or to understand the big picture of the puzzle.
Just remember, the more pieces of the puzzle you give your assistant, the sooner and better your assistant can put together the puzzle. Or even better, is give your assistant an idea of what the puzzle should look like when it is together so she can help put some of the pieces together for you.
Good luck!