The 2nd Ed. of AD&D greatly expanded the classic game, and this book is a must for playing 2nd edition. However, when TSR published the revised, 2nd edition books, they added material to the classic rules, lifted, and copied them into new volumes. Some things were not copied, unfortunately, and they are buried, and scattered, throughout the books, both PHB, and DMG. I keep my 1st Ed. books stacked with my 2nd Ed. books, and between the two, I have everything I need.
I almost did not purchase the 2nd Ed. books, when they came out -- I had been playing for 7+ years, and I had not even scratched the surface of what is possible within the game. Why would I want to buy this new edition -- what did I need it for? I purchased the PHB first, and after scanning the new ideas on character sub-classes, and specialized priests, I was sold! I loaned my PHB to two of my players, and that was all it took -- my entire group switched from 1st, to 2nd, and we never looked back!
Third Edition never intrigued me. Fourth Edition seems to be more focused on miniature gaming, which I enjoy as a separate, but closely related, hobby. Neither 3rd, nor 4th has interested me enough to even buy a single book to check them out... I am a die-hard fan of 2nd Ed., and I doubt I will change my interests, no matter what. This is one of the core volumes of 2nd Ed. games, and I highly recommend it, as part of the definitive edition of D&D, bar none. If you prefer role-playing, to rules-playing, and miniatures gaming is not the forefront of your RPG desires, then 2nd Ed. AD&D is precisely what you are looking for.
There are many, many books, from 2nd Ed. rules, which are still available. This edition of AD&D came out in 1989. It is now 2011, 22 years later, and the game is still quite popular, it is still being played, in spite of 3.0, 3.5, and 4.0! It is, in my opinion, the very best of the true role-playing, fantasy games. Cheers!