What is the best book on subject, how to decide products margins?
Seriously though, there's no book. Do what @Empires said. Figure out what the market will pay. Then figure out what you can buy for. Optimize both. That's your margin.
Also:
STOP LOOKING FOR BOOKS FOR EVERY QUESTION YOU HAVE.
Books are worthless for a majority of business questions. Instead, just take action and get the real answers that no book could ever tell you.