There are real gems among posts and articles people write on the forums or on their blogs. It takes some time to find them but when you do they are simply amazing read. I have two of such for you today. Just links to respective forums or blogs:
Hinge writes in a very informative but also very humorous way about the different roles of the fast cavalry a player may find useful to use. The inspiration to write it was the discussion on Orcs & Goblins forum where people discussed the current state of Warhammer and drew the conclusion that there is no space for fast cavalry in the game. Obviously, Hinge didn't agree but he was kind enough to write what this particular unit is good for.
The article written by KillerK is more detailed and discusses examples where Dark Riders are featured but the principles he is talking about are applicable to any other fast cavalry unit. He provides diagrams that illustrate the case studies very well and I think it is one of the best tactical articles for 8th edition Warhammer.