Since we went there on the 1st of Jan., National Library was closed. I guess, again, you'll have to wait for my next visit to Vienna. Sorry!
Very impressive building dated back to the 18th century. National Library contains an inestimably valoable collection of more than two milion handwritten and printed works.
The edifice occupying one end of Heldenplatz is the Nationalbibliothek, which means of course the National Bibliothek :)
I would say that the "österreichische Nationalbibliothek" is the nicest libary I have ever seen. I thought it was some royal house and it turned out to be a libary.