I have a Kindle Keyboard with firmware 3.4 and have encountered the same problem with the samples of Snuff (A Discworld Novel) by Terry Pratchett and Devil Said Bang (A Sandman Slim Novel) by Richard Kadrey. I believe they are KF8 files and not Topaz because they are very recent publications and Calbre reads them as AZW3. There is no option to turn the publisher font on/off, and switching between regular--condensed--sans serif changes nothing. When I contacted Kindle support they said it was the publisher's fault. The publisher never responded to my inquiry.
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