This book is an excellent introduction and guide to derivatives. It's also a good reference - there's enough information so that you probably won't feel you need to buy another, deeper book.
The emphasis is on a description of the various products, as well as how to price them. The emphasis stays on the theory of pricing, rather than delving right down into computer algorithms, so if you need to write derivatives software, I recommend the excellent 'C++ Design Patterns and Derivatives Pricing' by Mark Joshi - but read this book first.
This should definitely be your first book on derivatives. Only after reading several other books on derivatives, after first reading 'Hull', did I realise how clear his book is, and how some other authors manage to make things so confusing.