In the pyramid, opposite the hidden room with the last tome in it, there is a chest above the door.

I finally managed to get at it - stacking 4 chests on top of each other (not that easy to do in the first place and secondly at the right place).

Does anyone know a better way to get at that chest? (By the way, my wife did not even notice that chest in her run.)

You can enter the room's mezzanine level after much wending and winding around the pyramid. You would have to jump from one mezzanine section to the next to get to the chest though.

Thanks for the answer, but NO, I have been to the mezzanine (easy to find) and (at least at my level athletics [6+]) I cannot jump across - so that way does not work at all (for me at least).

Try looking up when jumping.
For these jumps, just running while looking up is good enough to cross the gaps, as long as you keep hugging the wall.