One thing I didn't realize when I started pitching the first book was that there would be "passive income" in addition to the book advance and royalties.
For instance, I've had three foreign-rights deals for "Experimenting With Babies," and the only additional work I had to do was put my signature on some paperwork.
And I have Amazon referral links on both websites, pointing to each book on Amazon. Those links generate a monthly average of about $40 in commissions per month, although my monthly high has been as much as $630.