Anisah,
Documents viewed in Arbortext Editor are not allowed to have multiple root elements.
Thus if your file entity is <book>...</book> and you try to insert into a <book>...</book> document, then it will fail.
For example in axdocbook you have a template file
<?xml version-"1.0" encoding="UTF-8"?>
<!-- Arbortext, Inc., 1988-2015, v.4002-->
<!DOCTYPE book PUBLIC "-//Arbortext//DTD DocBook XML V4.0//EN" "axdocbook.dtd:>
<book lang="en">
<chapter>
<title></title>
<para></para>
</chapter>
</book>
The file entity for the whole of the sample document would be
<book lang="en">
<title>PTC Employs......</title>
...
</book>
One cannot insert the fragment into the template document without making it out of context (inserting it somewhere within <book> and </book>), and
it cannot be inserted before <book> or after </book>, because it would violate the rule having to have a single root element.
Does that make sense?
It might be better to create file entities for chapters. Those may then be inserted with <book> and </book>.