DOCBOOK: What is included in a <set>?

[Tom Olsen <olsen@lsil.com>]

I'd like to make a <set> of our books so that links between them will work.
TDG gives the following example:

<!DOCTYPE set ...>
<set><title>The Perl Series</title>
<setinfo>...</setinfo>
<book><title>Learning Perl</title> ... </book>
<book><title>Programming Perl</title ... </book>

...

</set>

Question #1: What appears where the ellipses are between <book> and </book>?
Can I include the current book file via entity? Should I include the entire
contents of the book file? The whole book?

Question #2: Does the DTD Declaration in the <set> file supplant the DTD
Declaration in each book file?

Thanks in advance.

Tom Olsen