The version of the Book Tag Set (or Collection Tag Set) that was used to make the document instance under consideration
Note that this is a fixed value that should only be changed when the Tag Set changes versions or revisions, and must be changed in the DTD or schema. Like attributes with defaults, the value of the attribute is assigned to a document during parsing or other XML processing. The @dtd-version attribute cannot be changed in a document instance to a value that does not match the DTD/schema.
<!ENTITY % dtd-version "dtd-version CDATA #FIXED '3.0'" >