We prefer the DK Eyewitness guide as well, but I agree that you should test drive a few at your local library to see which format you prefer.
March is a delightful time to visit the country. Depending upon your available time and personal interests I suggest devoting a large portion of your discretionary time to the South Island and particularly the area in and around Queenstown.
One note - during our most recent visit last year we noted the crowds in some of the popular towns had increased dramatically as compared to previous trips. Apparently, NZ has become quite popular with Asian tourists during the Chinese New Year celebrations, which in our case lasted well into March. Rooms were hard to come by in Queenstown, Wanaka, and Mt. Cook. Point being that I'd be sure to lock in your accommodations early to avoid possible disappointment.