I've been less than pleased with this book. There have been lots of times where I expected a theory to have a brief mention but Heywood has left it out. In a theory book you really want as much as possible because otherwise once you've grasped the very basic fundamentals this book just sits on the shelf and you don't bother to check it because it's so unlikely it will have anything of use.
What is there is good, the clear indexing makes life a lot easier and the compact size has its advantages over some bulkier text books. The book is light and manageable. Heywood certainly has a talent for these sorts of minor reference and he writes well.
It's not a bad starter book but if you really want to be familiar with political theory you'd probably be better skipping over this, spending a little more money and buying something a bit more thorough so as to avoid exasperation and disappointment early on.