I've toured twice (1987 & 1997) in NZ, each time on both islands. While my favorite touring region was the west coast of the South I., as well as the northern part of the South I., there is some great touring on the North I. as well. Don't sell the North I. short. With two months, you'll have time to explore much of the country. One area which disappointed me was the east coast of the South Isl. from a bit north of Dunedin to Christchurch, though Christchurch itself is a pleasant town, as is Dunedin. I enjoyed the Otago peninsula just south of Dunedin, and the Albatross colony and yellow-eyed penguins there are well-worth seeking out, if that is still possible.
Wellington is a pleasant city with frequently awful weather, and both times I passed through there I took a bus between Wellington and points north on the North Island. The area immediately north of Wellington is relatively crowded, has more traffic than most of NZ, and I think is best avoided. The first time on the North I. I biked from Rotorua south to Wanganui, going on the west side of Tongararo (sp?) National Park. I enjoyed that route, despite being repeatedly dive-bombed by a territorial magpie south of Raitihi! The 2nd trip, I biked most of the route from Napier (my favorite town in NZ due largely to its fantastic art deco architecture) around the East Cape, up to the Coromandel peninsula just south of Auckland. The area is fairly remote and distances are great, but it's a pretty area. I had repeatedly heard that the Coromandel, so close to Auckland, was both very pretty and had some difficult cycling. Both proved to be true. Really lovely scenery there, but some difficult riding at times. Overall, I found the terrain of the North I. makes for more challenging riding than the South I. I've never been north of Auckland, and only one cyclist I've spoken with who had been there thought it made for good touring, though visitors otherwise liked the region.
NZ roads tend to be paved ("sealed" in local parlance) with coarse stones, so the roads aren't that smooth. However, the roads were otherwise in good condition.
My last trip I stayed mainly in "backpackers", and found the guest feedback ratings in the directory to be accurate and very useful. The first trip involved mostly camping and bunk rooms in the "motor camps".
In 1987, there were relatively few touring cyclists and it was easy to hook up with others. I biked with cyclists from 4 different countries on that trip. In 1997, the number of touring cyclists had increased at least 10-fold. There were so many cyclists that you no longer stopped and chatted with others you would pass on the road. If you did so routinely, you wouldn't get very far each day. I was amazed in 1997 how many cyclists there were from northern Europe. Most of the increase I observed seemed to be especially from Germany, but also Switzerland, the Netherlands, and Scandinavia. The number of North American and Australian cyclists had remained largely constant. In some of the backpackers in 1997, the managers were German, most of the guests were German, and the lingua franca was sometimes even German. I realized how non-native English speakers must feel at times during travels throughout the world.
If you enjoy hiking ("tramping" in NZ-ese), it is superb in NZ. I hiked the gorgeous Routeburn track, as well as hikes in 2 or 3 other national parks on both islands.
Personally, I don't think cycling guidebooks are necessary for NZ. The number of roads is limited. Once you determine what regions you want to visit, you don't have much choice where to ride. I guess the cycling books (I bought a couple) can be somewhat useful for deciding which regions you wish to visit, but I think that a general guidebook is more useful.