How to Show Metadata in the Cover Page
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Suppose you need to present the author and the ISBN just under the publication title. Your bookmap contains:
<bookmap id="taskbook">
    <booktitle>
        <booklibrary>Retro Tools</booklibrary>
        <mainbooktitle>Product tasks</mainbooktitle>
        <booktitlealt>Tasks and what they can do</booktitlealt>
    </booktitle>
    <bookmeta>        
        <author>Howe Tuduit</author>
        <critdates>
            <created date="1/1/2015"/>
            <revised modified="3/4/2016"/>
            <revised modified="3/5/2016"/>
        </critdates>
        <bookid>
            <isbn>071271271X</isbn>
            <booknumber>SG99-9999-00</booknumber>
...

The entire <booktitle> element content is displayed on the first page of the PDF, so suppose you need to add the information after it.

In your customization css, add the following CSS rules:
*[class ~= "bookmap/booktitle"]:after {
    display:block;
    content: "by " oxy_xpath('//*[contains(@class, " bookmap/bookmeta ")]/*[contains(@class, " topic/author ")]/text()');
    margin-top: 4em;
    text-align: center;
    color: gray;
}
*[class ~= "bookmap/booktitle"]:after(2) {
    display:block;
    content: "ISBN "oxy_xpath('//*[contains(@class, " bookmap/isbn ")]/text()');
    text-align: center;
    color: gray;
}
The following CSS extensions were used in this example:
  • oxy_xpath - Executes an XPath expression and returns a string content.
  • :after(N) - Creates more than one after pseudo element. The argument value represents how far the generated content is from the real content. In the example above, the content of the :after is closer to the title (upper) than the content of the :after(2).

Generating Synthetic Pages for Metadata.

Suppose you need to show this information on a page that follows the title page, instead of on the title page. In this case, you need to prepare a named page and place the content on it. In your customization css, add the following CSS rules:

@page page-for-meta {
    background-color: yellow; /* Just to see it better*/
    @top-left-corner {
        content:""; /* Remove the default header */
    }
    @top-right-corner {
        content:""; /* Remove the default header */
    }
}

*[class ~= "bookmap/booktitle"]:after {
    page: page-for-meta;
}
*[class ~= "bookmap/booktitle"]:after(2) {
    page: page-for-meta;
}