I've not read a huge amount, but Harrington's 6-max book is still pretty decent for the improving 6-max online player (although I disagree with some of his reasoning on pre-flop). Otherwise, there's a couple of micro specific books that everybody recommends that will make you profitable pretty quickly, but I forget their names now - no doubt somebody else will chip in with the titles.

The book I've personally valued the most is Matt Janda's, but this could be quite difficult to understand for a relatively inexperienced player and it won't help you beat up the micros so much. Definitely worth adding to your collection at some point though. Jared Tendler's Mental Game of Poker will always be worth buying too if you have problems with tilt.

Books to avoid imo: The Mathematics of Poker for now, as it's incredibly dry.

BUT: I'd still recommend joining an inexpensive video maker like Grinderschool early on, rather than buying books. They'll explain key concepts to you and talk about how to beat each of the levels in a clear way, along with excellent live play videos. Carroters in particular is worth watching.