Sorry but you DON'T already know everything about LBJ. True the man had his faults but despite my disagreement with most of his politics he was a master politician. In addition, Caro has found much new (or forgotten) information about LBJ. His article in the current New Yorker is an example. It shows the complexity of LBJ in regards to Civil Rights. Did you know about Johnson's private Chauffer? I think not. Did you know the episode about the burial of a Mexican-American veteran in Arlington Cemetary that LBJ had to back away from after initially instigating it? I think not.
Your attitude seems to be "I don't like LBJ so I won't read a bio of him even though it is masterfully written."
This attitude is like reading only feel-good news. Much can be learned by reading about LBJ and others that you (and I) might not agree with.