Using FLWOR to get data from another table

I have a table in SQL Server 2012 with this structure:

CREATE TABLE [dbo].[tblStepList]
(
    [ToDoId] [int] IDENTITY(1,1) NOT NULL,
    [Data] [xml] NOT NULL
)

And the Data column is of type XML and contents like this:

<Steplist>
  <Step>
    <StepId>e36a3450-1c8f-44da-b4d0-58e5bfe2a987</StepId>
    <Rank>1</Rank>
    <IsComplete>false</IsComplete>
    <TextReadingName>bug-8588_Updated3</TextReadingName>     
  </Step>
  <Step>
    <StepId>4078c1b1-71ea-4578-ba61-d2f6a5126ba1</StepId>
    <Rank>2</Rank>
    <TextReadingName>reading1</TextReadingName>
  </Step>
</Steplist>'

I want to update each row of the table with my new xml to look with new node named TextReadingId after TextReading name

I have a secondary table with text reading values stepid and textreadingid

StepId                                TextReadingId 
---------------------------------------------------
e36a3450-1c8f-44da-b4d0-58e5bfe2a987    118
4078c1b1-71ea-4578-ba61-d2f6a5126ba1    119
d466a8ee-9214-4b9c-94f9-2117f5dffe93    401

And I want my TextReadingId values to come from the table above

 <Steplist>
          <Step>
            <StepId>e36a3450-1c8f-44da-b4d0-58e5bfe2a987</StepId>
            <Rank>1</Rank>
            <IsComplete>false</IsComplete>
            <TextReadingName>bug-8588_Updated3</TextReadingName>    
          <TextReadingId>118</TextReadingId>   
          </Step>
          <Step>
            <StepId>4078c1b1-71ea-4578-ba61-d2f6a5126ba1</StepId>
            <Rank>2</Rank>
            <TextReadingName>reading1</TextReadingName>
          <TextReadingId>401</TextReadingId> 
          </Step>
        </Steplist>';

This is what I tried but it is not working as expected

DECLARE @i int;

SELECT
    @i = s.data.value('count(/Steplist/Step)', 'nvarchar(max)')
FROM 
    tblStepList s

SET data.modify('insert <TextReadingId>{sql:variable("@i")}</TextReadingId> as last into (/Steplist/Step[sql:variable("@i")])[1]')

PRINT @i

Here is another answer that helped but it does not join with my other table for the results

Solution

The easiest way to do this is to rebuild each Step node and then aggregate it back up using FOR XML

Within the correlated subquery, we do the following:

Break out the Step nodes using .nodes()
Left join StepReading on the StepId node value
Create an unnamed column containing a new Step node...
... which contains all the children from the existing one using ./* ...
... and an extra child node TextReadingId with the value from StepReading
Then aggregate back up using FOR XML

UPDATE sl
SET Data = (
    SELECT v.Step.query('
<Step>{./*,
        if (not(./TextReadingId)) then
            <TextReadingId>{sql:column("sr.TextReadingId")}</TextReadingId>
        else ()
        }
</Step>
    ')
    FROM sl.Data.nodes('/Steplist/Step') v(Step)
    LEFT JOIN StepReading sr ON sr.StepId = v.Step.value('(StepId/text())[1]','uniqueidentifier')
    FOR XML PATH(''), ROOT('Steplist'), TYPE
)
FROM tblStepList sl;

SQL Fiddle