 Solution

Inside LinkedListBook, traverse without modifying the head or any node:

def printBook(): Unit = {
  var p = list
  while (p != null) {
    println(p.name + ":" + p.number)
    p = p.next
  }
}

The loop invariant is that precisely the prefix before p has been printed, once per node, in list order; p heads the remaining suffix. The null-terminated linked list invariant proves termination and prevents duplicate output through cycles. An empty list prints nothing. The abstraction function is unchanged, and the traversal takes $O(K)$ node visits for $K$ entries.