issue quest The myth (the deadspell storm)

Hi
I finish this quest and the red book is above npc head, I can answer the right question to finish and then nothing happen.
I think there's a little bug here. 
Is someone finish it? if yes I ll glad to know how

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Edit :
it seems I don't have a potion bottle...

edit 2 : I bought an empty bottle to the merchant, but it doesn't work...

the purchased bottle must be the only one in your inventory afterwards, I think that's the only way it works.

It works, thanks