>>For each inserted book information into the Publications table this trigger also updates Books table and increments NumPublications field.
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>>In another words, say, you inserted two records into Publications table with the same ISBN, then Books table would have NumPublications field incremented by 2 for this Book.
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>Though does ISBN uniquely identifies each publication or not? In another words, if the same book is published by two separate Publishing companies, would it have the same ISBN or different?

Have no idea, but what if the same Publishing company release new edition of the book? Will it be with the same ISBN or not?