Should I let my professor know that I cited a text written by him....that I pirated from online
It is incredibly uncommon to assume that someone bought a book just because it is cited. While in theory, the professor could ask this question, why would (s)he care?
If you are geniuinely concerned about the question, perform an inter-library loan with your university library on the two books now, and should that question ideed pop up, reply that you performed an inter-library loan.
Use legal sources in the future. Libraries spend tons of money on buying literature, so use it. Most jurisdictions also allow limited copying of a book (e.g., the most relevant pages for your own work).
It may also be worth pointing out that your professor is unlikely to really care that you ‘pirated’ the book. While he or she will receive occasional royalty cheques, very few textbooks make more than beer money for the author.
They might object to the non-standard download on principle (and that's reasonable enough), but I'd be surprised if they made a big deal of it, even if they bothered to ask whether you'd bought the book (which, as others have mentioned, is unlikely).
That said, they quite possibly would be annoyed if they found that the university library didn't have a copy of the book.
Your use of the material should be covered under Fair Use. This doctrine provides for the legal, unlicensed citation or incorporation of copyrighted material in another author's work. However, there was no need to resort to unethical means to obtain the material.
Honestly, had you told you professor what you told us - that the subject poorly addressed in other papers - and asked to borrow a copy, I'm sure he would have been delighted and probably given you some brownie points.