When prospective clients tell me they'd like to write a book, one of the first things I ask is whether they are passionate about the topic — passionate enough to spend months (and often years) wedded to an idea that demands writing, perhaps research, then revision and more revision and more revision.
If they are unsure or want to test their commitment to the concept, I suggest they brainstorm 100 questions about things that fascinate them the most in life. The questions don't need to be written down in full sentences; just a line or even a few words will do.
What people often find is that the list has common themes or issues related to their proposed book project. Other times, however, people discover that their book project is not related to any issue that deeply interests or fascinates them, which means they are likely to lose interest when the going gets tough. And it always does. That's the kind of thing that's better to find out sooner rather than later.
Compiling a list of 100 questions also is helpful for writers trying to develop an outline or think about new ways to approach an existing project. It may be difficult to begin the list at first, but usually people are on a roll by the time they reach 50 questions. By then, their minds are wandering in more freewheeling and creative ways, and it becomes easier to come up with even more questions than they thought possible. Such is the magic of brainstorming.