An identifier, such as a DOI or archive control number, for a work
The pub-id-type attribute may be used to identify the type or source of the identifier.
The <isbn> which contains the ISBN for a work, a particular type of identifier.
<!ELEMENT book-id (#PCDATA) >
Text, numbers, or special characters
<book>
<book-meta>
<book-id pub-id-type="other">handbook</book-id>
<book-title-group>
<book-title>The NCBI Handbook</book-title>
</book-title-group>
<edition>1st</edition>
<contrib-group>...</contrib-group>
...</book-meta>
<book-front>...</book-front>
<body>...</body>
<back>...</back>
</book>
bookmeta.ent