If you don't want to sort the list, for whatever reason, you could always use a Max Heap structure with a heap size of 100. Iterate over your books and, if the new book has higher sales than a book in the heap, replace the bottom element with the new one. This assumes that no two book sales are going to be the same, so you would have to consider that particular wrinkle. In .NET for instance, you can use the PriorityQueue class to handle max heaps.
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