Hull is the one stop shop for learning (and using as a reference once you have learnt "it") almost any topic in Quant Finance. This book really has it all - options, interest rate models, volatility modeling, credit derivatives, you name it. However, use it as an introductory book - learn the main concepts from here and then move on to advanced books. As a current student of quant finance, I find Hull's book to be indispensable.

I recently entered the field of quantitative finance from physics, and this book has been by far the best resource as both a learning tool and reference. Its bredth of coverage is outstanding, covering topics as simple as bond yields and zero rates, to sophsticated libor market models, weather derivatives, bermudan swaptions, etc. While a bit wordy at times, Hull does present all the mathematics in a tractible manner. Books with more mathematical sophistication, such as Brigo & Mercurio, while interesting, are far less useful on the trading floor.

With each chapter Hull presents several "Business Snapshots" providing practical real life illustrations. Overall, this is a great purchase someone with a quantitive background new to finance.