Hello Ganesh,
There is a good SCN posting by Jogeswara Rao Kavala last year regarding the retrieval of long texts. Please search SCN for "Retrieving Longtext" and it comes up as first in the list. It is entitled "Retrieving Longtext-lines for Reporting" and it will walk you through the use of the function module READ_TEXT. I hope this helps.
Best Wishes,
Bob Pennington