'If you read Professor Harry's book, you should be able to recognize and successfully attack fallacious reasoning, and you are able to do that. Therefore, you must have read Professor Harry's book.' Which formal fallacy is committed in the above argument?

The argument affirms the consequent.

Its form is: if you read the book (P), then you can attack fallacies (Q).

The premise affirms Q, that you can attack fallacies.

From Q it concludes P, that you read the book.

The ability could arise otherwise, so P does not follow.

Inferring P from a conditional plus Q is affirming the consequent.