The real rewards of writing are in the act of writing itself and in the lives that you affect with your work.
Selling books is not the motivating factor for me. For me, editing is very fun. Sometimes I will get an inspiration from the works of other writers. I will then go back and try to improve a phrase or sentence, or using my imagination to create a more descriptive metaphor. I have spent hours looking for individual words that convey exactly what I want to say. Sometimes I will spend a whole writing session researching data to formulate what I think is a one perfect paragraph.
Most of all, the feedback that I have received from people whose lives I have touched with my writing is what makes it all worthwhile.