True, this is not valid. 
How about changing [pdf.xsl](https://github.com/spring-projects/spring-data-build/blob/master/resources/src/main/resources/docbkx/xsl/pdf.xsl) for `authorgroup/autor` from `<xsl:value-of select="." />` to 

````xml
<xsl:value-of select="firstname"/><xsl:text>&#160;</xsl:text><xsl:value-of select="surname"/>
````