t3488g, I have a pile of Osprey books, but I don't have their Churchill one. ("It's not my bag, baby", - Austin Powers) For $15, they are a good overview of the vehicles presented, with development, variations, service histories, several color plates, a cut-away drawing and lots of photos, with an average of around 50 pages.
As for the mud thing...Not necessarily more area needs to be covered in mud, maybe just an embellishment of what you already have on there. If you look again at your reference photo, you have the mud in the right places. Those guys are really churning it up, but if you look at the sides of the vehicles, they are fairly clean, just as you have modeled it. My thought was just to add a little more substance to it, like by adding some sand in the mix, to make it thicker and chunkier.