Before reading (i.e. studying) this book, I thought Hardy's and Apostol's books on the theory of numbers were the best... Those are very good indeed but Gauss' treatment is that of a MASTER !
Once you get used to the symbols, the journey is thrilling : you're dealing with a genius, willing to guide you...
Don't throw your money away in buying the paperback edition, buy the hardcover edition where most of the errors have been corrected and which is much better structured and provides ample space for annotations... i.e. allow yourself a gift and enjoy it !
Gauss's Disquisitiones is among the most revered classics in all of mathematics. This translation published by Springer makes it available to English speakers. It isn't cheap and I imagine it will be purchased mainly by professional mathematicians, historians of mathematics or academic libraries. Either you want a copy or you don't. I happily forked out the readies for it. What more can one say?
Well, there is a *lot* more one can say. Here the reader will find the original statement of the concepts and notation of arithmetic congruences and the celebrated proof of the law of quadratic reciprocity. Here one finds what is still one of the best accounts of number theory in existence. Despite being two centuries old, this is still a strikingly modern exposition, such was the genius of the author.
Gauss has a reputation for being terse and pithy. Clarke, the translator, deliberately carries what he calls Gauss' "Ciceronian" style into English and rightly so. That said, one has to recall that Gauss often erased all traces of the processes of insight that led him to many of his results. Hence his work is not easy reading even for professional mathematicians. Nevertheless, nobody seriously interested in number theory should go long without reading this work. It is a founding document of contemporary mathematics and anyone calling himself a mathematician has his claim diminished if he has not read it.