What is the root node in XML?

I was reading an e-book and had the following xml code.

<?xml version="1.0"?>
<?xml-stylesheet href="sonnet.xsl" type="text/xsl"?>
<?cocoon-process type="xslt"?>
<!DOCTYPE sonnet [
  <!ELEMENT sonnet (auth:author, title, lines)>
  <!ATTLIST sonnet public-domain CDATA "yes"
      type (Shakespearean | Petrarchan) "Shakespearean">
  <!ELEMENT auth:author (last-name,first-name,nationality,
      year-of-birth?,year-of-death?)>
  <!ELEMENT last-name (#PCDATA)>
  <!ELEMENT first-name (#PCDATA)>
  <!ELEMENT nationality (#PCDATA)>
  <!ELEMENT year-of-birth (#PCDATA)>
  <!ELEMENT year-of-death (#PCDATA)>
  <!ELEMENT title (#PCDATA)>
  <!ELEMENT lines (line,line,line,line,
      line,line,line,line,line,line,line,
      line,line,line)>
  <!ELEMENT line (#PCDATA)>
]>
<!-- Default sonnet type is Shakespearean, the other allowable -->
<!-- type is "Petrarchan." -->
<sonnet type="Shakespearean">
  <auth:author xmlns:auth="http://www.authors.com/">
    <last-name>Shakespeare</last-name>
    <first-name>William</first-name>
    <nationality>British</nationality>
    <year-of-birth>1564</year-of-birth>
    <year-of-death>1616</year-of-death>
  </auth:author>
  <!-- Is there an official title for this sonnet? They're
  sometimes named after the first line. -->
  <title>Sonnet 130</title>
  <lines>
    <line>My mistress' eyes are nothing like the sun,</line>
    <line>Coral is far more red than her lips red.</line>
    <line>If snow be white, why then her breasts are dun,</line>
    <line>If hairs be wires, black wires grow on her head.</line>
    <line>I have seen roses damasked, red and white,</line>
    <line>But no such roses see I in her cheeks.</line>
    <line>And in some perfumes is there more delight</line>
    <line>Than in the breath that from my mistress reeks.</line>
    <line>I love to hear her speak, yet well I know</line>
    <line>That music hath a far more pleasing sound.</line>
    <line>I grant I never saw a goddess go,</line>
    <line>My mistress when she walks, treads on the ground.</line>
    <line>And yet, by Heaven, I think my love as rare</line>
    <line>As any she belied with false compare.</line>
  </lines>
</sonnet>
<!-- The title of Sting 1987 album "Nothing like the sun" is -->
<!-- from line 1 of this sonnet.

The problem is that when I continue reading, I thought I misunderstood what the root element is. According to my value, the root of the node is equal to <sonnet>node.

But in this book I find the following

Unlike other nodes, the root node does not have a parent. This always takes place with at least one child element of the document. The root node also contains comments or processing instructions that are outside the document element. In our sample document, two processing instructions called xmlstylesheet and cocoon-process are children of the root node, like the comment that appears before the tag and the comment that appears after the tag.

, . , ? , node.

?

<?xml version="1.0"?> 
<Food> 
  <Cateogry> Vegetable</Cateogry>
  <Cateogry> Fruits</Cateogry>
  <Cateogry> other</Cateogry>
</Food>

? ?