Please help: Can't change typeface in Kindle3

[tilacog]

Hello guys!
I'm having a strange problem with the typeface of some of my Kindle 3 books. I hope someone could help me on this:

For some books, Kindle uses this monospaced typefont (looks like Courrier New), and I can't change its type (only its size).

Strangely, they are all .mobi files, originaly converted from .epub files by Calibre, where they typefaces have a fine display, even after the .mobi conversion.

For the many other books I did the same process, the problem cited above doesn't exist.

I was wondering if someone else had a similar problem, or could help in any way.

Thanks in advance!