This book, at almost 900 pages, is probably as detailed a one book examination of LBJ's life as we're likely to get. Of those pages, about 50% of the book focuses on the Vice-Presidency and Presidency, 30% on his work as a Congressman, Senator and Government Representative, and about 20% focuses on his early life.
To be honest, I found the book somewhat depressing. It describes a man, who because of his poor upbringing and origins (in the American South), couldn't be the man he probably wanted to be (and stay elected).
I also found the book surprising. I was born only a little before Johnson died, but always imagined him as decisive, because he seemed so effective in the Senate. It seems, from reading this book however, that you'll be presented with a guy who feels he's lacking in something, especially when compared to the JFK brigade in the White House.
The other thing that surprised me is that he strated disliking some aspects of the Great Society, almost as soon the were passed. As an example, one was targeted at getting inner city parents to stay together. Almost immediately, it became clear it wasn't working, which resulted in LBJ grousing about the law regularly.
There has been some some criticism of the accuracy of statements in the book. As an example, it says RFK was assassinated at the Embassy Hotel (no he wasn't, it was the Ambassador Hotel), and that certain Senators and Governors have been designated as representing the wrong state. I would agree with some of those criticisms, but I don't necessarily think those errors reduced the book much, because they weren't major. A southern anti-integration Senator is a southern anti-integration Senator, irrespective of whether you have him down as being from Alabama, or Mississippi.
I will say, however, though that the errors reduced the credibility of the book some, as it makes you wonder about the accuracy of the research.
In short, I think most people will get something new out of the book, if you can ignore the errors. I enjoyed reading it, and not just because I have a soft sport for Texas, and the area Johnson grew up in.