You've figured out what all the knobs and dials and various menu options are on your camera and you're ready to take your photography to the next level... Neil van Niekerk's On Camera Flash is a good place to start!
And now you've played with bouncing your flash and have gotten great results, but you're looking for a little more freedom to create... it's time to take your flash off camera. Off camera flash might seem overwhelming at first, but Niekerk breaks it down in such a way that the information feels digestible and accessible.
Direction & Quality of Light is a wonderful companion to Niekerk's other books and a beautiful place to start learning how to see and use light to create stunning images.
When you're ready to gain a deeper understanding of light and get just a little bit more complicated, check out these books from Joe McNally.