But I'm wondering what the equivalent would be for conditional map
entries.

Supposing that there is an additional optional attribute on the 'book'
element of the original xml, 'out-of-print="true"'.
Furthermore, let's say that the JSON maps in the 'books' array contain
an additional field when a book is out of print.

In XSLT map entries, you can write:

<xsl:map>
B B B <xsl:map-entry key="'title'"B B select="title!string()"B B B B

B  B  />
B  B  B  <xsl:map-entry key="'authors'" select="array { ./author!string()
}" />

B  B  B  <xsl:if test="@out-of-print andB @out-of-print eq 'true'" >
B  B  B  B  B B <xsl:map-entry key="'out-of-print'" select="true()" />
B  B  B  </xsl:if>
</xsl:map>

But I'm thinking that there is not a way to express this with the 'map
{ ... }' form in xpath.

map {
B  B  'title'B  B  B  B  : title!string(),
B  B  'authors'B  B  B  : array { author/string() },
B  ( 'out-of-print' : true() )[@out-of-print and @out-of-print eq 'true']
}

Is there any way of doing something like this using in-line xpath?

B  <xsl:template match="/">
B B B  <xsl:sequence
B B B B B  select="map {
B B B B B B B B B B B B B B  'books' : array {
B B B B B B B B B B B B B B B B B  books/book ! map {
B B B B B B B B B B B B B B B B B B B  'title' : title!string(),
B B B B B B B B B B B B B B B B B B B  'authors' : array { author/string() },
B B B B B B B B B B B B B B B B B B B  'out-of-print' : @out-of-print =
'true'
B B B B B B B B B B B B B B B B B  }
B B B B B B B B B B B B B B B  }
B B B B B B B B B B B B B  }"/>
B  </xsl:template>

i.e. give any map representing a book a property of the name
out-of-print with the right value.

B  <xsl:template match="/">
B B B  <xsl:sequence
B B B B B  select="map {
B B B B B B B B B B B B B B  'books' : array {
B B B B B B B B B B B B B B B B B  books/book !
B B B B B B B B B B B B B B B B B B B  map:merge((
B B B B B B B B B B B B B B B B B B B B B  map {
B B B B B B B B B B B B B B B B B B B B B B B  'title' : title!string(),
B B B B B B B B B B B B B B B B B B B B B B B  'authors' : array {
author/string() }
B B B B B B B B B B B B B B B B B B B B B  },
B B B B B B B B B B B B B B B B B B B B B  @out-of-print !
map:entry(local-name(), boolean(.)))
B B B B B B B B B B B B B B B B B B B  )
B B B B B B B B B B B B B B B B B  }
B B B B B B B B B B B B B  }"/>
B  </xsl:template>