Indicates the unique identifier for a book, so that the book as a whole may be cross-referenced. IDs must be unique inside their document (not just within their element type), and the XML parser enforces this.
The ID and IDREF attribute values constitute a primitive reference mechanism for XML. For example, when an @book-id attribute has been used on a <book> element, that identifier can be referenced by an IDREF-style attribute on a cross-reference <xref> element, so that the full book can be pointed to.
An identifier string is typically 1-32 characters in length and starts with a letter of the alphabet. (It is an XML rule that identifiers may not start with a digit.)
|An identifier (ID)||A unique identifier for the element. XML parsers can verify that an element pointing to one of these is pointing to a valid ID.|
|Restriction: This attribute may be specified if the element is used.|