Topaz is a book format that is occasionally used for Kindles. It is very similar to a PDF, but tailored a bit more for small screen readers like the Kindles. Some older books were printed but there is no digital file available for them containing the text. So the publisher basically scans the book and creates a Topaz formatted Kindle book from the scans. They are usually horrible to read.

I suspect the book you bought is simply one in which the dopey publisher tried to force a specific font via CSS (cascading style sheets).