Adding books in small batches isn't necessarily anything to do with any problems your Kindle might have indexing a large amount. It's to do with the drain it puts on the battery, and more importantly, if for whatever reason there is an issue with a book and your Kindle either gets stuck indexing it (and therefore keeps draining the battery) or starts freezing up, it's easy to go back and find which book is causing the problem if you've only added a small number at a time.
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