I wouldn't go crazy, because the vast majority of books you can just get from the library as and when you need them. However I own tonne of books because I'm overly keen haha.
Aulton's Pharmaceutics and Rang and Dale's Pharmacology are two essential books, as has already been mentioned. They're probably worth the investment in your own copy. The FastTrack series of books are good for quick reference and revision, so maybe consider buying a few of those later in the year. We were also told to buy a medical dictionary, but that's not an essential purchase straight away, so I'd wait to see what your tutors say.
And of course you NEED a BNF! What is a pharmacist if they don't have a BNF? Some universities give them out for free, mine certainly did. If not though, the most recent edition can be bought for a reduced price of £20 if you're a member of the Royal Pharmaceutical Society (student membership is free, and you'll probably get signed up during Freshers week).