"Product specific, market specific, or company-wide plan that describes activities involved in achieving specific marketing objectives within a set timeframe. A market plan begins with the identification (through market research) of specific customer needs and how the firm intends to fulfill them while generating an acceptable level of return. It generally includes analysis of the current market situation (opportunities and trends) and detailed action programs, budgets, sales forecasts, strategies, and projected (proforma) financial statements.
Whew! That's a lot of words.
Basically, it's a plan for your marketing. When you plan a trip, you have a goal of where you want to end up then you decide how you will get there. It's the same with marketing your books. Who are you trying to reach? What do you want to do to reach your goals? How much time do you want to take? How much money do you have to invest?
Don't solely think about your plan. Write it down. Make it something solid. Develop solid numbers you want to reach in activities, people reached and your end goal (book sales). Write down your budget. Have a calendar to set time limits on each task.