It is quite amazing how many people want to write a book. One of the first things I ask them is, "What's your book about?" That's when the talk starts, even from the people who are already well into the process of writing. I usually get some sort of long winded explanation of the problems, solutions, opinions and personal philosophy of the would-be writer.
The reason I ask this question first is because if you cannot answer it before my eyes glaze over from boredom, then you are not writing anything that someone will read. A brilliant business coach that I know says that explanations kill deals. It is certainly true in the book writing business. The answer needs to be one sentence long and leave space for the listener to be curious enough to ask another question, instead of remarking about the weather or your long lost second cousin - anything to get you off whatever the subject of your book is.
Many first-time writers write the book first and then try to find an answer - it is backwards. Answer the question "What's your book about first?" with a one sentence answer and the whole writing process goes forward much easier and you won't bore your party guests to tears. Interestingly enough (true confession time) I did not know that when I wrote The West Coast Trail: one step at a time or I would have made it much stronger in the adventure department and left the preparation and trail guide stuff behind. Of course, the fun thing for me now is that I get to speak about the adventure to various groups and the trail information is a bonus instead of the focus.