There is not a standard commission amount for a book agent to take from an author of a book, but the average is about 10 to 15% for domestic sales and then 20 to 30% for foreign or co-agent sales. Click here for this website for more information on literary agents and what to look for in a good versus a bad agent. This is through the Science Fiction and Fantasy Writers of America, but most of the information should be able to go across genres. Here are a few more websites that you may find helpful in this matter (click on the highlighted areas to bring them up):

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Please let me know if you need any further information on this.

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