If the font is ok, then probably the problem is with the encoding - which is set per book. In 0.8.14 (e.g. on a Linux Desktop) I think there is a "language" tag (there definately is on 0.8.17) which allows setting of the default encoding and an option for automatic encoding detection. However, I don't know how these are represented in ui.xml.
Have you tried converting to a format that includes the encoding? This may be as simple as using HTML with charset=utf-8:
<meta http-equiv=Content-Type content="text/html; charset=utf-8">