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Like everything in this crowded world, the rules are there for safety and to minimise trouble for other passengers.
You should have the minimum amount in your cabin baggage. Necessary valuables and money, passport and tickets. Also whatever you need on the flight. A great many lockers are stuffed full by the time it gets to take-off, and I'm not the only one who has received a direct hit from falling oddments!
Baggage not securely stowed will move up and down the flight by itself. I was on a short business flight, and we had three briefcases coming down the central aisle. The owners could do nothing till the seat belts light went out. Red faces when they went in search of their cases.
So as you can realise, when the seat belts light is off, you can retrieve your stowed luggage, but remember that the overhead locker may be a little difficult to reach, so just before you board the flight put your passport, money and book into a pocket. Don't forget that you will be stowing an outer coat or jacket, so that's not immediately accessible all the time.
Enjoy your flight