Hm, it'd certainly be possible (heck, probable) harm for the company if their books were leaked ahead of time, so I'd say it'd be morally questionable to download them... but practically speaking, unless the entire internet refused to look at the books, the cat's already out of the bag, and all the harm's already been done.
Since it's a victimless crime that doesn't harm society (because, well, you're not harming WotC's sales beyond what already legal methods allow you to do, you're just doing it at home), I don't see a moral fault in downloading books post-release for preview purposes only. Of course, it should still be illegal, because unlike a preview reading in a bookstore, it has very easy and obvious possibilities for misuse.