How do I use someone else's trademark for a product line I want to make?

[William]

I own a small business and I have an idea for a product line that uses fictional characters from a well-known book to make it interesting and appealing to the market. Since they are a registered trademark that belongs to the author/publisher how exactly to I use them? I suppose I'd have to contact them and negotiate some terms, but how would I get in contact with them to begin with, and then what?