The file OEBPS/Styles/style.css within the book indeed contains:
Code:
body.especial { font-family:"Times New Roman", Times, serif; margin:1em 0 1em 1em; }
which is bad coding. You could remove the font-family part (with the calibre book editor, for example), or you could try overriding it by adding "!important" to the body font-family:
Code:
body {
font-size: 8pt;
font-family: Bookerly, serif !important;
...
}