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Best payment processor for normal business i.e. book sales.

If your business is legit (which I'm sure it is) then you may look into payment service providers like Online Payment & Credit Card Processing | WorldPay, Wire Card, 2checkout, Netbilling, Ogone Payment Services - Seamless & secure payment solutions and simlar services.
 
In order for us to give you any sort of advice or direction, we need you to clarify as to whether you are selling e-book downloads or hard copies and shipping to people. If you are selling e-books or digital downloads, what is the billing/shipping model you currently use? This will determine how easy or complicated this will be.