Ummm with Star Wars the films have always stayed the constant core. But the material in the books and the comics has been reshaped all the time. Lets not forget these are just fan terms. The huge amount of "retroactive continuity" done with SW stuff would be called a reboot anywhere else. What they need to do to fit all these inconsistencies together is very similar to what DC comics does. They reboot some stuff fully and keep other stuff. So the word "retcon" is used to explain how it can fit.
The point is in order to understand how Zahn's version of the Clone Wars fit with the real versions movies you have to track down magazine articles and other books that later written later . The new movies are going to largely be aimed at people who have seen the first 6 episodes only! You got to wonder if at some point Lucasfilm is going to want to open Star Wars tie in books to everyone. Not a small group which have studied everything published for 35 years so they alone can understand how it all fits together.