How much should I give a percentage to someone who helps me write my book?

An amount proportionate and commensurate with the proportion of the work he did. Between the two of you, you could decide that such and such an amount is for the creative part of the process then simply divide the rest along percentage lines all the best urban

You should have agreed on the amount beforehand. Ask her personally how much she expects from you for helping in writing the book. Her fee is something the two of you shall set. If you find that her asking price is much too big, ask for a discount - just negotiate so the amount becomes acceptable to you; if it is very low, discuss it with her; if it is already at acceptable levels, take it. Put the agreement into writing so no one among you forgets what had transpired during the meeting of your two minds.

It depends on his work! Anyway, less than 25!