From a review in the New York Times (my emphasis):
What makes Mr. Darman’s book so compelling is his focus on the roughly 1,000 days between Kennedy’s assassination and the midterm elections in 1966, which not only encapsulated the rise and fall of Johnson but also the emergence of Reagan as a national political figure. Johnson easily swamped Senator Barry Goldwater in the 1964 presidential election, but middle-class disenchantment with the Great Society and growing unease with the Vietnam War combined to produce a stinging rebuke in the 1966 midterm elections. (The Democrats kept control of Congress, but the Republicans picked up 47 seats in the House and three in the Senate.)
Personally, I learned three interesting things about Johnson (Reagan, as many authors have found out, remains unknowable):
1. The president had two beagles named Him and Her;
2. The Johnson administration was much more pessimistic about Vietnam and much earlier (almost from the beginning) than I had thought; and
3. LBJ almost declined to run for reelection in 1964 even though he was riding high and crushing Barry Goldwater in the polls. The 36th president, though publicly optimistic, was something of a defeatist in private. This episode, in which he was talked down off the ledge by his wife, foreshadowed his decision not to run in 1968.
The book also touched on LBJ's stormy relationship with Bobby Kennedy and piqued my interest in the subject. I just put two books on reserve at the library, Robert Kennedy: His Life Mutual Contempt: Lyndon Johnson, Robert Kennedy, and the Feud That Shaped A Decade by Jeff Shesol.
* Is Mr. Darman the son of Richard Darman, who served under Presidents Reagan and George H. W. Bush? (If so, he writes objectively -- I can't tell what his politics are.) In the elder Darman's Wikipedia page it says he had a son named Jonathan W. E. Darman, but I can't tell if it's the same person. I tweeted the question to Darman just now; I'll let you know what I find out. (Aren't you on pins and needles?)
Turns out, Jonathan Darman is indeed the son of Richard Darman. The author reveals this in the second to last sentence in the Acknowledgements section.