How to search a particular field in lucene.net

The error message tells you what's wrong: The stored procedure expects a parameter @Booknum which you did not supply with your SqlCommand. Instead you add an unnamed SqlParameter with no value - which doesn't help because the stored procedure expects that named parameter. Also, you need to provide the SqlParameter before you execute the SqlCommand (by calling ExecuteReader() on it).

It might work if you do this:

cmd.Parameters.Add(new SqlParameter());           // remove this line
cmd.Parameters.AddWithValue("@Booknum", Booknum); // (note the quotes)
SqlDataReader reader = cmd.ExecuteReader();       // line moved

Edit: You should move this line:

con.Close();

..below the if-block in which it currently is in:

if (reader.HasRows)
{
   // ...
}
con.Close();

..because you want to close the SqlConnection in any case, not only if there are no results.

Even better would be to use the SqlConnection with a using-Statement because this will ensure that the connection gets closed (and disposed) in any case, even if an exception is thrown somewhere "inbetween" - and the same goes for all other Sql****-objects like the SqlCommand, SqlDataReader, SqlTransaction (and others):

using (SqlConnection con = new SqlConnection(connstring))
using (SqlCommand cmd = new SqlCommand(sproc, con))
{
    con.Open();
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@Booknum", Booknum);

    using(SqlDataReader reader = cmd.ExecuteReader())
    {
        List<Document> docs = new List<Document>();
 
        if (reader.HasRows)
        {
            // ...
        }
    }

    // no explicit closing of the connection required any more
}