A quick search shows that the 7th book in the Harry Potter series is 2Mb in size so providing there aren't lots of photos or diagrams in it, not a lot.
Storing one photo is likely to take up more space Text files tend not to have large storage requirements, even less if they are compressed as most compression algorithms are pretty good on plain text - 25% of original size is not uncommon.
Books can easily go over 100mbs if they have lots of illustrations. Magazines are typically 15 - 70 mbs. But if its mostly just text and in epub format then books can easily be less than 1mb.
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