I just gone through my PLR articles collection which covering niches such as dog trainng , gardening ,health and fitness , home schooling , car buying , debts, bonsai , wine , boat , fishing , speed reading , acting , stamp collection ,camping and etc.... If you not doing any black hat adSense and actually read through each of them ,imagine the amount of knowledge you will gained from them.
Learning while Earning from Google AdSense , does it make sense to you ? It does make a lot of sense to me :)