If you want to further your knowledge of wine then one course stands head and shoulders above the rest - the WSET (Wine Spirit Education Trust)
Their Foundation course is fairly basic, but the intermediate and advanced start to get you prepared for a knowledgeable future. (then into diploma - and on to Master of Wine)
Well written, clearly divided up into easy sections and easy to follow. Perhaps not as simple to follow as the 'Dummies' books, but deeper and ultimately more rewarding.
If you are planning on doing the WSET course then it is essential - no other book will do it - learn this and you will pass. If you are not following the course - still worth buying, interesting and very informative.