One way to market this product without spending money is to contact publications that dentists are likely to read. Explain to the Managing Editor that you'd like to do a Joint Venture with their publication, where if they place an ad in their publication, promoting your book, you will pay them a percentage of each sale generated. You can track sales by having a tracking number in the bottom corner of each order form. In each publication that agrees to do this, the only thing that would need to be different in each ad would be the tracking number on the order form.
The best way to ensure your success with this is to make it as easy as possible for the publication to participate. Provide them with a ready-to-go ad, include a brief but complete description of your proposal and MOST IMPORTANTLY - give them a generous cut of the profits (at least 50% of the net profit)!!!
This method has been used by some of the most successful marketers and has literally made millions of dollars in sales without the marketer having to spend a dime on advertising!
A spin-off to this idea is determining your exact cost to produce and ship each book. Then, add one or two dollars to that amount. The amount you come up with will be your "shipping and handling charge" Tell the publication that you'd be willing to provide their readers with your book for free if they place an order form in the publication. The only thing their reader would have to pay for is the shipping and handling. Publications love being able to give things away to their readers. You'd pocket $1-$2 profit for each copy you "gave away". It might not sound like a lot, but I know a software marketer who used this method and "gave away" over 300,000 copies of a CD ROM and he made $1.50 on each order he shipped out!
Hope you found some of these ideas useful!