Dave Chesson has been around a a good long while and he keeps on top of the trends as they change. I just reviewed a batch of his videos last night and learned something new; definite value in those. (A friend urged me to buy Publisher Rocket last year before the price went up, and I did, but I confess that I have not used it yet.) Even if you don't buy anything from Dave, you can work his methods yourself for free. What Rocket does is compile many more stats than you or I would ever have the patience to collect. And some that we may or may not need depending on where we are in our publishing journey.
I would listen to this Dave, David Gaughran, and Bryan Cohen when it comes to marketing. They spend most of their time learning the ins and outs. (Also, Nick Erik, but he's mainly for people deep in the trenches of significant advertising spend.) That doesn't mean that following their particular pieces of advice will always work for your particular books. YMMV, as always.
