Negative dialectics is obviously one of the most important texts for 20th century philosophy, and does not really require any introduction (aside, perhaps, from a warning of the difficulty of some of the passages - you need to know a fair amount about Hegel and Kant to understand it...).
The important thing is getting the right translation: whatever you do, do NOT get the Ashton one. It is fast becoming notorious, if for no reason other than the sheer quantity of blunders it makes. Entire passages are left pretty much unreadable (not to mention the violence done to the work of a stylist as subtle as Adorno). The Continuum version is, in my opinion at least, a far better bet, and to top it all off, it's cheaper too (both in terms of the literal price of the book and the fact that you won't have to buy a copy of the German original in order to work out what's going on).