Is there a Forgotten Realms sourcebook that details Toril's Outer Planes? If not, are there any good online resources that compile canonic information?
Well, these seem to change from edition to edition. For 3e, I know there are a few pages in the 3e FRCS. But Candlekeep is a good place to look for resources (or to ask questions in the forums) about all things FR. There are many knowledgeable posters, and many of the authors and designers also read and respond.