The Next Book?

I had to change some of the chapter names again - apparently as soon as this was done the whole thing goes into editing. Quite exciting, and I suppose a landmark moment.
Now I have to think about my next book. Knowing publishers (who like their authors to keep writing the same thing), I will need to do another travel book, and I do have an idea planned. I have actually run the idea by my publisher, and she loves it. The problem is, until my first book is released and proves a financial success (which I am confident about), I seriously doubt the publishing house will give me another advance to do the relatively expensive - and extensive - travel that the new book will require.
But I want to keep writing, and working on a serious project, just to keep the momentum up and keep me in that psychic space of "being a writer."
So for now I am working on a project that I call Spiritual Journeying. It has elements of the travel genre in it, but all the travel is being done in my own city, or even closer to home - my own head. Basically it is a manifesto for the spiritual dilettante, an account of my own, unashamedly syncretic, spiritual journey. I have already written a substantial part of the first chapter, which is on the organised Interfaith movement, but so far have been to shy to show it to my publisher. Mostly because I feel certain that she will reject it and tell me to wait for the next travel piece. But I am still working on it, because I am passionate about it, it excites and interests me, and it seems the perfect thing to be writing about.
Is it the Next Book? I don't know. But it's my new lifeboat, the thing I can point to when people ask me what I am working on right now.