I started reading the Monster of Florence last night. It's by no means dense, but I did find it relaxing to read right before bed. Peaceful even. It made me realize I haven't read for leisure in so long (thanks college). I'm curious to see how this will affect my RC.I think simply reading is hugely beneficial. It doesn't have to be Kant; just pick up a book (or Kindle) instead of turning on Netflix every night and you'll make enormous gains in your reading ability. The sad fact is that many of us are addicted to our phones, social media, etc., and are constantly multitasking, so it's difficult for us to switch back to focused text-processing mode and concentrate on anything longer than a page. Reading almost anything will help; the important thing is to read and read a lot.
For that reason, I'd recommend that OP read whatever is interesting to him. It's better to read 1000 pages of fluffy material before the test than 50 pages of dense philosophy.
However, if he wants something close to RC passages, I'd suggest the old standbys the Economist and Scientific American, or a contemporary philosopher like Rawls. No RC passage will look like anything in Plato or Homer.
I think I'll start reading the Economist during my lunch breaks at work. Thanks for the advice!