The concept of companion planting is a great one, however I would advise against purchasing this as your only book on the topic. It is not a 'reading book', as it is set out in an A-Z reference fashion, with a quick guide to good/bad companions at the back, and no index, so if you want to remind yourself about a term you came across somewhere you will have to reread virtually the whole book to find it.
As others have noted, sometimes terms are confusing (something about a caterpillar worm comes to mind, but I can't find it now (no index!) but most of all the book is appallingly edited. There are spelling mistakes, whole paragraphs repeated (eg. see the entry on Sage) and downright contradictions (Fennel is listed in both the good and bad companions for coriander!).
Unless I was unlucky enough to receive the only badly edited copy of this book, then I am amazed that it has received such praise. Yes, 5 out of 5 for the concept of companion planting, but not for this poorly presented piece of writing. A quick A-Z reference would be invaluable if it was reliable, and I feel the author had the right idea, but something went terribly wrong between there and publishing.