Merging multiple XML files in PowerShell

I'm running a script in PowerShell to try and create one big XML file, it's actually very similar to this question: PowerShell XML Nodes from Another File

To that extent I'll borrow the code from the answer.

foreach ($file in $files)
{
    [xml]$filecontents = get-content $file

    $entity = $fileContents.Model.LobSystems.LobSystem.Entities.Entity  

    $importedEntity = $master.ImportNode($entity, $TRUE)

    $master.Model.LobSystems.LobSystem.Entities.AppendChild($importedEntity);

}

The main difference for me is that the variable entity returns a NodeList so I cannot use ImportNode.

Here is an example piece of XML that illustrates the point:

<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid Studio -->
<bookstore xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:noNamespaceSchemaLocation="BookStore.xsd">
    <book price="730.54" ISBN="string" publicationdate="2016-02-27">
        <title>string</title>
        <author>
            <first-name>string</first-name>
            <last-name>string</last-name>
        </author>
        <genre>string</genre>
    </book>
    <book price="6738.774" ISBN="string">
        <title>string</title>
        <author>
            <first-name>string</first-name>
            <last-name>string</last-name>
        </author>
    </book>
</bookstore>

Where in each file there are multiple Book items, so if I did the following:

$entity = $fileContents.BookStore.Book 

I'd get a NodeList.

Is there a way to loop through a NodeList and append each item as a node?

I also tried the following:

foreach ($file in $files)
{
    [xml]$fileContents = get-content $file

    $entity = $fileContents.rss.channel.item

    $importedEntity = $master.ImportNode($entity, $TRUE)

    $master.rss.channel.AppendChild($importedEntity);
}
$master.Save("New-Master.xml")

Where the entity I want is a little deeper but the same principle applies.