Conditionals in XML file using xmlstarlet

Question

Considering the codes in here and here, I've changed them to explain my question. Now the code looks like this:

<?xml version="1.0" encoding="ISO-8859-1"?>

<bookstore>
<book category="COOKING">
  <Description>
    <title lang="en">Everyday Italian</title>
    <author>Giada De Laurentiis</author>
    <year>2005</year>
    <stock>YES</stock>
  </Description>
  <Location>
    <restock>UMG</restock>
    <shelf>30</shelf>
  </Location>
</book>

<book category="CHILDREN">
  <Description>
    <title lang="en">Harry Potter</title>
    <author>J K. Rowling</author>
    <year>2005</year>
    <stock>NO</stock>
  </Description>
  <Location>
    <restock>GIP</restock>
    <shelf>20</shelf>
  </Location>
</book>
    <book category="CHILDREN">
  <Description>
    <title lang="en">Harry Potter</title>
    <author>J K. Rowling</author>
    <year>2015</year>
    <stock>YES</stock>
  </Description>
  <Location>
    <restock>GIP</restock>
    <shelf>21</shelf>
  </Location>
</book>  
<book category="WEB">
  <Description>
    <title lang="en">XQuery Kick Start</title>
    <author>James McGovern</author>
    <year>2003</year>
    <stock>YES</stock>
  </Description>
  <Location>
    <restock>NGT</restock>
    <shelf>11</shelf>
  </Location>
</book>

In my personal issue, is that I want to first, check if a book is in stock, and if it is, check the shelf where it is located. Access to the value of stock if pretty straight forward:

xmlstarlet sel -t -c "/bookstore/book/Description[stock='YES']" book.xml

But I am unable to do the conditional. In the xmlstarlet guide, it says the I should use -i or --if, but I've been trying to do it as follows:

xmlstarlet sel -t -c -i "/bookstore/book/Description[stock='YES']" -v "/bookstore/book/Location/shelf" book.xml

Because I saw a similar problem, but it is now working. Any ideas?

EDIT:

Using the following approach, I don't get errors, but anything at all

cat book.xml | xmlstarlet sel -t -m "/bookstore/book/Description" -i "@stock='YES'" -v '/bookstore/book/Location/shelf'

EDIT 2:

I've been thinking about what would happen if I have two books called the same. I've edited the code above, and now I have 2 book called Harry Potter, each one with different publishing dates and shelves

Following Daniel Haley approach, I want to know all the book with the title harry potter:

 xmlstarlet sel -t -v "/*/book[Description/title='Harry Potter']/Location/shelf"

But I only get the first result, but I want all of them.

Answer 1

In addition to not needing conditionals (-i), it's also not really necessary to match (-m) either; just get the value (-v)...

xmlstarlet sel -t -v "/*/book[Description/stock='YES']/Location/shelf" -n book.xml