Oh, and hey: many people do claim that the USSR wasn't actually _socialist_. You'll find two groups who do this: anarchists, and new Marxists who still have some liberal left in them. The USSR certainly claimed to be socialist, and was according to socialist principles.
The two groups will make this incorrect, revisionist claim because of two reasons. Anarchists want to bolster socialism while trashing statism, so they want to play up 'state socialism's failures. New Marxists will want to play down any perceived 'failures' of Marxism by distancing themselves from said 'failures.'
If you only want one book, that book is Capital I. I suggest David Harvey's lectures along with it, they'll help out quite a bit. If you have read Smith and Ricardo, that'll make it much easier, too. For bonus points, read the short "Wages, price, and profit" by Marx, it's a bit easier of an intro than the book's.
If you can fit in a second, I'd suggest "the State and Revolution" by Lenin.
Marx identifies a problem and a crude solution, and Lenin provides a good solution. Those two will give you the most of it. Wikipedia, especially on the historical/dialectical materialism stuff, is helpful.
A more full list is here: http://www.reddit.com/r/communism/comments/wisiw/basic_marxi...
Also, /r/communism101 is very good and there to answer questions, if you Reddit.