This is a legal question, not a tax question, but I can tell you the IRS position on releasing information:
Without proper consent, the IRS cannot release your information. I cannot get any information about my clients without a signed power of attorney, for example. By going online, your landlord sidestepped the authorization using your tax return information and posing as you. At best, XXXXX XXXXX highly unethical.
The purpose of you showing your landlord your tax return was to prove you were receiving a refund. Verifying this should have been a non-issue. Your landlord obviously doesn't trust you (or perhaps anybody for that matter). If your landlord uncovered information that did not agree with your tax return, which seems to be the only reason he would let you know what he did in the first place, then hell, you may have been fraudulent yourself, in which case you two deserve each other.
I'll ask again now in this thread, as your question does not give every possible detail whether you like it or not. Did you sign a rental agreement that perhaps contained some fine print and authorized your landlord to do things like obtain credit reports, tax transcripts, etc.? If so, it may be perfectly legal.
If all you did was hand your landlord your tax return, but did not sign anything, did not give any verbal authorization, etc., then it sounds like what he did was illegal. Now you need to go to the lawyer's forum, as this is not a tax question any more.
My thoughts are that, if there were no actual damages caused by your landlord, you're not going to win anything in a civil suit (the ones where greedy idiots sometimes try to get money, and of course, sometimes good people use them legitimately also). A criminal charge is different, but doesn't seem too beneficial to you, so what does it matter. I'm not a lawyer though, so I don't really know the answers to the legal stuff. This whole question seems absolutely ridiculous...