New (EPub) Ebooks Have Different, Less Readable Typeface

I recently purchased three new O'Reilly ebooks.

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When I view the EPub versions of these books in Calibre or in Firefox EPUBReader, the typeface is different--and less readable--than in O'Reilly ebooks that I've purchased in the past, such as Learning Vi or C# 4.0 in a Nutshell.

Using Calibre's settings, I'm able to change the typeface that is used for monospace text, but I cannot change the typeface for serif text which is the content that appears to have degraded. It almost seems as though the serif typeface is built into the document.

Could recent builds of ebooks be using a different serif typeface than previous builds?

I'm attaching a couple side-by-side screenshots that show the difference in typeface between the previously-purchased ebooks and the newly-acquired ones.

I do not see the issue when I view the ebooks on my iPodtouch using Stanza.