Nokogiri block assignment

gistfile1.txt

[myxml]
<item>
<title>Book1</title>
<author>Bob</author>
<link>http://url1.com</link>
</item>
<item>
<title>Book2</title>
<author>Bob</author>
<link>http://url1.com</link>
</item>

doc = Nokogiri::XML(fh)

#insert thumbnail for selected authors.
retval=(doc.xpath("//author[text()='Bob']/..")).each do |item|
  media=Nokogiri::XML::Node.new "media:thumbnail", item
  media.set_attribute("url","http://url/file.jpg")
  author=item.at_xpath "author"
  author.add_next_sibling(media)
end

puts doc.to_xml #returns all nodes + Bob's thumbnails
puts retval.to_xml #returns nothing

How to assign the matching block value to an array?