T-SQL: Selecting the top n rows from a Group By

I have the following data

deal_tracking_num	tran_num	type_id	value	personnel_id	last_update
897693	897693	20065	Manual	20015	2012-11-22 10:59:38.240
897693	897693	20065	Non-STP	20015	2012-11-22 11:09:09.190
897695	897695	20065	Manual	20015	2012-11-22 10:58:58.063
897695	897702	20065	Manual	20015	2012-11-22 10:59:23.493
897698	897698	20065	STP	20015	2012-11-22 10:16:23.903
897698	897698	20065	eCM	20015	2012-11-22 10:18:38.587
897698	897701	20065	eCM	20015	2012-11-22 10:21:07.350
897698	897703	20065	eCM	20015	2012-11-22 11:22:25.377

To select the first row for each deal_tracking_num, use the following query:

select a.deal_tracking_num, a.value from 
(
     SELECT ab.deal_tracking_num, ati.*, ROW_NUMBER() over (partition by deal_tracking_num ORDER BY ab.deal_tracking_num, ati.last_update) as rank
     FROM ab_tran_info_history ati JOIN ab_tran ab ON ab.tran_num = ati.tran_num
     WHERE ati.type_id = 20065 -- Trade Confirmation Type
) a
where RANK = 1

This returns

897693	Manual
897695	Manual
897698	STP