The always helpful Academic Coach published a post recently on how to start thinking about writing a book.

We write books for job security, naturally. But is knowledge served by enforcing one book per tenuring or promotion? Books written on the fly, while teaching courses, in five years?

But after we have tenure, why should we write books? Maybe tenured professors should stop publishing books so that there's room for junior faculty to publish books for tenure. But it's all backwards: the tenured faculty have all the time in the world to publish books, to write great books, whereas untenured faculty don't have that luxury.