In addition to what Talespin said: I thin BLO for one reason, it's just easier to handle. The application and wiping off are both just a little friendlier when it's thinned. That's one of the reasons I thin varnish (I don't use polyurethane varnishes, only alkyd or phenolic resin formulas). Besides the leveling out stuff mentioned it makes it a little easier. As for your question about learning more, I highly recommend Bob Flexner's book. It's easy reading, I read mine much like a novel just starting in the front and going through. Another choice that has the same info arranged in a different fashion would be Jeff Jewitt's book. I find his to be a little more technical (on formulations and the finish chemistry) and more suited to being used like a reference book, where you look particular subjects up. Both are excellent, and I think one of the them, or both, should be required material in any hobbyist WW'ing shop.