Somewhat surprisingly to me before I did it: if you control the marketing/sales channel, then it is rather substantially a better deal that you might think it is after the launch window. The reason is that the book remains in the market after being written (like a software product and unlike, say, a consulting gig), and you can structure your affairs such that people are routinely exposed to it and you get decent residual income from it.

e.g. From my video course that launched last September-ish:

http://images1.bingocardcreator.com/blog-images/hn/power-of-...

My ballpark estimate for total sales was in the $20k to $40k region, which I would have allocated 95%+ to the initial launch window. Surprise x2: a) I underestimated how much it would sell and b) I greatly underestimated the residual value of the project.

(Incidentally: The long-term marketing/sales channels are partially ranking #1 for [lifecycle emails], a subject of interest to many marketers which honestly I should not be #1 on the Internet for, and also having a brief in-text commercial in one of the emails that people get a few weeks after they sign up for my email list. Most of y'all can execute on these, particularly the autoresponders for new signups for the email list.)