Dynamic SQL Help Needed.

TABLE IN USE:
CREATE TABLE [dbo].[Book](
    [FirstName] [nvarchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL,
    [Price] [decimal](18, 0) NOT NULL,
    [LastName] [nvarchar](50) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL
) ON [PRIMARY]
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This is the procedure I have made. It compiles successfully. 
ALTER PROCEDURE InsertDataIntoBook
@empdata ntext,
@columns varchar(4000),
@xpath as varchar(4000)
AS

Declare @SQL varchar(max)
DECLARE @ParmDefinition nvarchar(500)
-- SET @SQL = 'DECLARE @XML as nvarchar(max)' + CHAR(13) + CHAR(10)
SET @SQL = 'DECLARE @DocHandle as int' + CHAR(13) + CHAR(10)
-- SET @SQL = @SQL + 'SET @XML = @xmldoc' + CHAR(13) + CHAR(10)
SET @SQL = @SQL + 'EXEC sp_xml_preparedocument @DocHandle OUTPUT, @xmldoc' + + CHAR(13) + CHAR(10)
SET @SQL = @SQL + 'INSERT INTO Book ('
DECLARE @hDoc int

   DECLARE @pos        int,
           @nextpos    int,
           @valuelen   int

   SELECT @pos = 0, @nextpos = 1

   WHILE @nextpos > 0
   BEGIN
      SELECT @nextpos = charindex(',', @Columns, @pos + 1)
      SELECT @valuelen = CASE WHEN @nextpos > 0
                              THEN @nextpos
                              ELSE len(@Columns) + 1
                         END - @pos - 1
     
        SET @SQL = @SQL + convert(varchar, substring(@Columns, @pos + 1, @valuelen))
      SELECT @pos = @nextpos
      if @nextpos > 0
         SET @SQL = @SQL + ','
   END
SET @SQL = @SQL + ')'
SET @SQL = @SQL + CHAR(13) + CHAR(10) + 'SELECT * FROM OPENXML(@DocHandle,''/book'',2)'
SET @SQL = @SQL + CHAR(13) + CHAR(10) + 'WITH('

SELECT @pos = 0, @nextpos = 1

   WHILE @nextpos > 0
   BEGIN
      SELECT @nextpos = charindex(',', @XPATH, @pos + 1)
      SELECT @valuelen = CASE WHEN @nextpos > 0
                              THEN @nextpos
                              ELSE len(@XPATH) + 1
                         END - @pos - 1
     
        SET @SQL = @SQL + '[' + convert(varchar, substring(@XPATH, @pos + 1, @valuelen)) + ']'
        SET @SQL = @SQL + ' Varchar(1000)' + '''' + convert(varchar, substring(@XPATH, @pos + 1, @valuelen)) + ''''
      SELECT @pos = @nextpos
      if @nextpos > 0
         SET @SQL = @SQL + ','
         SET @SQL = @SQL + CHAR(13) + CHAR(10)
   END
SET @SQL = @SQL + ')'
Print @SQL
EXECUTE sp_executesql @SQL, N'@xmldoc ntext',@xmldoc = @empdata;
------------------------------------------------------------------------------------------------------------------------ 
@SQL is string that contains the dynamic sql. 
Using @xmldoc as an input parameter to dynamic sql. 
------------------------------------------------------------------------------------------------------------------------
Executing like this: 
EXEC    @return_value = [dbo].[InsertDataIntoBook]
        @empdata = '<book genre="autobiography" publicationdate="1981-03-22" ISBN="1-861003-11-0">
    <title>The seven Kingdoms</title>
    <author>
      <first-name>Benjamin</first-name>
      <last-name>Franklin</last-name>
    </author>
    <price>8.99</price>
  </book>',
        @columns = N'FirstName, Price, LastName',
        @xpath = N'//book/author/first-name,//book/price,//book/author/last-name'
---------------------------------------------------------------------------------------------------------------------
Giving this error :
DECLARE @DocHandle as int
EXEC sp_xml_preparedocument @DocHandle OUTPUT, @xmldoc
INSERT INTO Book (FirstName, Price, LastName)
SELECT * FROM OPENXML(@DocHandle,'/book',2)
WITH([//book/author/first-name] Varchar(1000)'//book/author/first-name',
[//book/price] Varchar(1000)'//book/price',
[//book/author/last-name] Varchar(1000)'//book/author/last-name'
)
Msg 214, Level 16, State 2, Procedure sp_executesql, Line 1
Procedure expects parameter '@statement' of type 'ntext/nchar/nvarchar'.
The dynamic sql string is build correctly but somehow it is not getting executed.

Thanks for reading through this long post.