Friends sometimes ask, "Where should I order your book in order to be most advantageous to you as the author?" That's a very kind thing to care about. Ultimately, whatever method gets the book into your hands is the method I want you to use.
That said, there are a number of options:
Chalice, my publisher, has the cheapest price online, at $15.99. I make more per book that at other online retailers. But shipping is additional, so you need to factor in that cost when you order.
Many people like Amazon for the Prime shipping, although they just raised the price of my book this week to $17.99. (I can't make heads or tails of what they do.) I make somewhat less per book through Amazon than through Chalice, but ordering through Amazon increases the book's rank, which (I think?) raises its visibility on topical searches and such. It's also possible that for a book as 'small' and specialized as mine, rank doesn't matter.
I also ship directly for orders of 10 or more. I'll sign them and include an invoice, or you can pay me through PayPal. I can usually do this for about $15 a copy. Email me at email@example.com if you want to pursue this.
As I said, it's kind for people to ask where they should order the book in order to yield me the maximum profit. But book writing is not a big money-making enterprise for most of us. And that's OK. My aspiration is to write and to be read. If you visit this blog regularly, then I've met my goal. If you buy the book and read it, that's icing on the cupcake.