[anatta]

Get "Inside the Mind.." by John Feeney. He has two good essays on the transition to the middle limits. Regarding tricky players, you can play a little more passively against them at first until you get your bearings, checking the turn more and check calling them (which might be the superior play anyways if they like to bluff). However, I wouldn't worry about it. If your game is ready, you will be able to identify the same huge mistakes in your opponents. Game selection as the other post said does become more important as you move up, but I suspect that 10-20 level isn't really that tough anywhere.