Few books can safely be called definitive - this is one of them. Shacklady and Morgan's magnum opus covers the history of the Spitfire in minute detail, showing not only the what but the why of every design change and modification. The book is at its strongest when covering all of the various marks of the Spit, and in the comprehensive listing of every Spitfire ever built - over 20,000 individual entries!
Its weak points? Well, one isn't really a weak point, more a matter of emphasis - if you're looking for stirring tales of combat, this isn't the book for you. It's far more concerned with the technical aspects of the design than in recounting all the details of its combat career. That's not really a problem, because the book states up front what it is about, and sticks to it. The only real weak point is that the quality of the photos in the Key edition is not as high as in the first edition, but as it's about half the price, that's a perfectly acceptable trade-off in my view.
If you have any interest in the greatest fighter ever built, this book is a must.