I think I can anticipate what you think is problematic about Caro's treatment of the 1957 Civil Rights Act: He's trying to make it carry more thematic freight than is justified, right? For one, Caro wants the episode to illustrate Johnson's skill. When the bill is being debated, particularly in the chapter "Yeas and Nays," Caro makes it seem as if Johnson is fighting against nearly impossible odds. And yet, earlier in the book, in his biographical passage on Richard Russell, he highlights Russell's brilliance and cunning by portraying the Southern senators' pro-segregation stance as doomed unless the senators played their hand absolutely perfectly. When Johnson presents his bill, suddenly the Southern bloc—now in tandem with the conservative Republicans behind California's William Knowland—looks invincible again.
For another, Caro wants the battle over the 1957 bill to illustrate Johnson's compassion: It's evidence of his compassion that he is willing to engage in such desperate horse-trading and to alienate his old base.
And yet, throughout, I'm struck by how woefully weak the case for Johnson's "compassion" is. Johnson once yelled at his substitute limousine driver, "Let me tell you one thing, nigger. As long as you are black, and you're gonna be black till the day you die, no one's gonna call you by your goddamn name. So no matter what you are called, nigger, you just let it roll off your back like water, and you'll make it. Just pretend you're a goddamn piece of furniture." Wow. This strikes me as a pretty silver-lining-less piece of outright abuse, the worst kind of Jim Crow recreational racism—made worse by the pseudo-pedagogical "listen-to-your-betters" condescension. Caro doesn't read it that way. He says that Johnson "used these epithets, and the pain they caused, in a different way, to teach the employee the lesson Johnson felt everyone had to learn, a lesson Johnson felt would lead to an improvement in the employee's life: that it was necessary to accept reality, to face harsh facts and push beyond them …"
It's good that Caro has recovered this incident, but he has misread it. I find him wholly gullible on the matter of LBJ's sympathy for have-nots. This is the volume where Johnson the "liberal icon" (a cliché, but a useful one here) begins to nudge aside Johnson the realpolitiker, so we're constantly running across passages like this: "And [Johnson, as a young teacher,] saw into his pupils' hearts. 'I saw hunger in their eyes and pain in their bodies,' Lyndon Johnson would say years later. 'Those little brown bodies had so little and needed so much.' " This is schlock. Johnson's straitened upbringing in the Hill Country gave him a vocabulary with which to describethe downtrodden of all races. But the evidence that it gave him a jot of sympathy for them is weak or nonexistent.
Now, having brought this up, I should immediately add that it doesn't bother me. That's because Caro—whom you're right to view as almost as power-drunk as his subject—never loses sight of the fact that, for Johnson, everything always came second to power. To change the subject for a sec, that's why I also agree with you that LBJ is ultimately unworthy of a biography of this depth. Power is one of the most important things in human life, but it is also the least subtle and the most boring. Power-mania renders Johnson's "appetites" and his "compassion" and his "single-mindedness" mere superficialities. I don't think I've ever read a biography of a more two-dimensional person; over the long haul, he emerges as infantile, petty, and tedious.
What happened to make Johnson change from a dug-in seg speaking weepily about "we of the South" to a champion of civil rights? Well, for one, Brown v. Board of Education. For another, the lynching of Emmett Till and a series of other last-ditch outrages rendered Northern opinion rock-solid against segregation. In short, what changed was the political climate, at just the moment Johnson began to think about the presidential nomination in 1960. The New Deal strategist Tommy Corcoran (whose readings of day-to-day events are consistently the most astute things in this book) describes Johnson's bizarre, eleventh-hour effort to throw his name into consideration for the presidential nomination in 1956, after his shilly-shallying had already sent his natural Southern allies over to Adlai Stevenson. Johnson took an important lesson from the obdurate refusal of Walter Reuther and all the Northern Democratic delegations to play ball with him. Says Corcoran: "He really thought these guys were going to come around. Hell, as long as he wasn't with them on civil rights, they were never going to support him!" So, when Johnson trolled the Southern Caucus for votes for the 1957 bill, saying, "We have to give the Negroes something," what he meant was, "You have to give me something"—to run on three years from now. And this is something about which Caro is always admirably clear.
Let me finish by answering the question with which you start your letter: How will Caro fit the remaining 13 years of Johnson's life (actually 16, since this volume wraps itself up rather perfunctorily after 1957) into the remainder of the series? As a practical matter, there cannot be three more volumes. Caro is approaching 70 and writing at a 10-year-per-volume pace. But there's also a problem with two volumes: Johnson's presidency is stuck dead-center of three uneventful years as veep and five uneventful ones as a Texas rancher. So, you'd have to split them in the middle of his presidency—say, in 1965—after the enactment of the civil-rights and social legislation, but before the escalation in Vietnam. My prediction, then, is another door-stopper, maybe even a longer volume than this one, which would come out around 2015. Maybe Slate will let us review it.