Equally important is to figure out what to write about. What you know best may not be what the potential readers want to read. As with startups, it is always good to some basic research about the market size. Amazon Sales Rank, a ranking based on sales velocity at Amazon.com and updated hourly, offers an interesting and useful tool to do so. Books with higher sales rank are selling much faster than the ones with lower sales rank.
The Sales Rank at any given time is available as part of Product Details section in Amazon listing. Historical values can be found at Charteous (http://charteo.us).