As several other questions have pointed out, graded readers are a good type of material for extensive reading. Where can I find graded readers in Norwegian? I am interested in finding them both in Bokmål and Nynorsk.
The closest thing I have been able to find is the Norwegisches Lesebuch. Lesestücke in der norwegischen Reichssprache, edited by J. C. Poestion in ... 1902. Since copyright on this book has expired, it is now in the public domain and available on Archive.org. The language it represents is Riksmål. According to Wikipedia, the latest reform of Riksmål in 2005 made it more or less the same as Bokmål, but I think the language has evolved since 1902, so it is not clear to me how well the Riksmål in the above book is similar to today's Bokmål.