The library had been closed for about three hours when firefighters were called to the scene. No one was injured.
Firefighters put out the flames but not before the library was heavily damaged.
The fire gutted the administrative area, and the rest of the building sustained severe heat and smoke damage.
Investigators say a burner switch on an old kitchen stove had been pushed in, and that caused the blaze.
Twenty-five thousand books must be sorted through volume by volume. The library staff has begun to box up those books that were relatively undamaged.
They will be moved to the old Harnett Primary School which could open as a temporary library in about a week.
That would be good news for both children and parents. Summer is one of the library's busiest times, and Dunn's summer reading programs have been delayed by the fire.
The library's computers are a total loss due to melting or water damage.
Meanwhile, head librarian Deborah Melvin and her staff suggest that checked-out materials be returned to other Harnett County libraries or left in the book drops.
Next week, a firm that specializes in restoring books after fire or other damage will come to Dunn to lend more hands to the project.