There's a place for consistency, and a place for difference in your graphic approach.

I recommend forming an overall graphic strategy before really investing in designs. The two big components of this are:

  • Figuring out your business's Brand Definition: This is the process of defining your business: Who you are, what you do, who you can best help, and what makes you different from your competition. You must have all of these factors well-defined to create an effective brand. If your thoughts in all of these areas aren't clear, concise, and well-put, then you can't hope to effectively direct your customers' thoughts about your business. Worse yet, you might wind up looking like you have "multiple personality syndrome," which can thoroughly confuse your customers.
  • Figuring out your marketing system. What pieces are you going to be creating/designing to get the message out about your business? How do they all fit together? For example, with this eBook, how will people be getting it? What do you want them to do after they get it? What other marketing materials will they see, and how can you keep everything consistent? At the same time, you don't want them to glance at your big products and think they already got them because the picture on the cover is the same. Differentiation in the graphics between your materials can make prospects able to see quickly all that you have to offer.