Tolkien's books and letters
 

OK I've already posted this in the Novices and Newcomers forum, but I felt it was not the right place so here it is again:
Moderators feel free to close the topic in the N&N forum.

A lot of information people on this forum use are from Tolkien's letters right?
The ideas Tolkien wrote down in his letters often vary from those in The Silmarillion, the Hobbit and LOTR.
I thought however that Tolkien was ever changing things in his books so they would be "perfect" after his death.
But how do you explain then that the books still differ from his letters?
Didn't he find those changes good enough to change them in his books? So what's the standard and where was Tolkien more happy with, details from his letters or his books?