I've tested a lot of resources through the years I've been in this business and below are some of my recommendations. Sign up here to get my weekly newsletter with book marketing tips.
KINDLE SAMURAI This is a very powerful piece of software that allows me to save hours of research in figuring out the most effective keywords for my books. (It only runs on Windows so if you have a Mac you need to get Bootcamp or VMware Fusion before you can install it). Here's a free tutorial on how I use this software to select the best performing keywords.
Learn how one author sold 3,500 copies of his book in one day thanks to a Facebook Event he hosted. You can do it too!
This is my email service provider. We all know that every author needs a mailing list. I've tested several of them before ending up with GetResponse which meets my needs best. If you need a tutorial on how to get yourself set up and how to get readers on your list, I go through this in detail in my Goodreads Cracked Training.