In Chapter 30 Radix Sort you say:
Radix sort is one of the fastest sorting algorithms. The average time complexity of radix sort is O(k × n), where k is the number of significant digits of the largest number, and n is the number of integers in the array.
Radix sort works best when k is constant, which occurs when all numbers in the array have the same count of significant digits. Its time complexity then becomes O(n).
Even if k is constant, though, you still have to do a full loop though for each significant digit. Are you sure you meant to say O(n)? It seems like it is still O(kn).