I'd have to say the Toronto Reference Library is one of the best public libraries in the city.
I thought it would be kinda noisy as it's very large and open however that's not the case.
If you can't find a book at school defiantly check the catalogue of TRL because they'll probably have it. This has happened to me on way to many occasions where the books I need have been taken out already at all the libraries at UofT so I checked the TRL and it saved my life.
What to expect when you go to the library... some of the libraries and people who work there aren't really all that pleasant but there are some who are really helpful. There's a main computer area on the main floor and a dozen on the other floors so if you need to use one and forgot you laptop you can hook that up with no problem. Not to mention since it's a reference library there's a whole bunch of photocopiers so if you can't stay you can just photocopy the pages you need.
If you do plan on staying to study, and like to study in quiet, then I'd suggest going to one of the higher floors or getting a private study room on the main floor. Most people are studying and grabbing books from the 1st and 2nd floor, so there can be some distraction. Just go up a floor or two and it's all good.