228,000 bytes / 1600 books = 142.5 bytes used per book on average for storage in the hash.

Subtract from 142.5 the average number of characters in the user ID AND the number of characters in an ISBN number. Then, you've got a reasonable estimate of the number of bytes of overhead that are used for each element in your hash.

Then, you can make some good guesses about the upper memory bound on your program!