An OpenCart beginner's guide book is availble for pre-order £36.93 ($63.18 USD).

Here is the description:

• Develop a professional, easy-to-use, attractive online store and shopping cart solution using OpenCart that meets today's modern e-commerce standards.
   
• Easily integrate your online store with one of the more popular payment gateways like PayPal and shipping methods such as UPS and USPS.
   
• Provide coupon codes, discounts, and wholesale options for your customers to increase demand on your online store.
   
• With hands-on examples, step-by-step explanations, and illustrations

You can purchase the book here:

https://www.packtpub.com/open-cart-1-4-beginners-guide-for-professional-online-shopping/book

There is also another book on its way about developing templates for openCart.