Nikon is known for their in-depth training with courses typically costing around $50 each. Through the end of May, Nikon is making much of their online courses free. The best part is that it isn’t completely Nikon oriented. There’s great info here no matter what brand you shoot.
Some Titles I Recently Perused
Beyond the Fundamentals of Photography
Discovering Macro Photography
Exploring Dynamic Landscape Photography
…and quite a bit more.