In reply to Rickrick:
I've used the Gå Telemark guidebook, the Rockfax miniguide and free online guide below.
Rockfax is the only guide with photo topos, although the route selection is very limited. Particularly when some routes (eg Mot Sola) can suffer from seapage. Some of the routes used for abseil descent are also omitted (eg Gone with the Weed). I would say it's the least useful for these reasons. The Rockfax topos are helpful for identifying obvious lines such as Mot Sola and Hägar. Otherwise once on the routes I've never had a problem with route-finding irrespective of which guide was with me. Many of the routes also have the names written on the rock at their bases.
This free online guide is very good:
http://www.osifjell.no/arkiv/klatreforer_nissedal.pdf
The Gå Telemark guide has the best coverage if you're willing to track down and pay for a copy. Particularly away from Hægefjell. It also shows the routes in relation to one another a little more, which helps for when trying to find an abseil. It uses a mixture of Nordic and UIAA grades to slighly confuse matters, however I've found it to be the most accurate for grading in spite of this.
I'd use either the free guide or Gå Telemark guide for the general coverage. Get the RF guide as well if you think the wide angle photos will help you find the base some of the routes. Also the stuff in English (although nothing essential to know). There's a bit to pay for parking and camping at Hægefjell so bring some hard cash.