This is far more than just another introductory book, as can be seen from the fact that has been referred to in judgements from the Law Lords of the Appellate Commitee of the House of Lords. ( For example, see Lord Steyn's judgement in Woolin, where he refers to Andrew Ashworth in two places, including a reference to page 172 of this book. )
This is a well written book from a highly regarded legal mind. Ashworth has done cutting edge work on sentencing, omissions and is the Vinerian Professor of English Law at Oxford University.
My only criticism is that some of the philosophical discussions in this book can be a bit daunting to a newcomer. I would suggest that you "cheat" a little and read this book along with one of the excellent books by Jonathan Herring.