"Blood, Money and Power" by Mark McClelland.
Also read "A Texan Looks at Lyndon" by J. Everett Hale. This was writtn in 1963-1964 by West Texan Hale and tells Lyndon's story well. He was the ultimate opportunist.
The author of Blood Money and Power was Barr McClellan [Scott McClellan's father].
Barr quotes a book written by LBJ's Texas mistress [I'm sure there were others in DC]. The mistress believed LBJ was involved. The night before the assassination she was with him in a public place, he went to a meeting, and made comments to her after the meeting. The title of the book was Texas in the Morning. I can't remember her name, but she died about 1999.