I have read the Harrington books Vol. I, II and the workbook several times now. Each time I read it I get more out of it. Those little details and little nuances that I may have glanced over on previous readings I am starting to pick up and starting to understand how important they are. Each time I get stuck at a plateau in development, I go back to the HOH books and can usually find the answers I am looking for.
As for Mase's post, I mostly agree with that: studying + playing experience = maximum benefit. One compliments the other. Also he is right about the microlimits. Some of the stuff he goes over (I.E. squeeze play) isn't going to be too effective at the lower levels. However, Dan is a very solid player (I don't think anyone is going to dispute that) and if you can get a good grasp of what he is trying to teach, you are going to have a firm foundation to build off of. From there you can start branching out and learn new stuff, like concepts found in 'Kill Everyone' (as mentioned by rStormChaser).
The book 'The Poker Mindset' I think is another good book. It does not deal with the mechanics of playing, but the mental aspects of the game. If you are learning the game I would probably put it on hold. Once you start taking bad beats and you start going on tilt frequently, that's when I would read it.
Lastly, the article written by Chris Ferguson on proper
bankroll Management should be at the top of your list. Takes about 10 minutes to read but will save you the heartache of trying to figure out what limits to play and when you should move up (or down).
Lots of great stuff out there. Good luck.