Since the first time Martha Washington strode into a Presidential abode (even before the White House existed), the role of the First Lady has been shaped by the women who held the title. Running the gamut from glorified arm candy and rabid redecorator to cause supporter to practical co-president, there is no one set mold to follow.

The rap on the former Claudia Alta Taylor by much of general history has been this: She was not as attractive, fashionable or glamorous as her predecessor, Jackie Kennedy. The courtship between her and her future husband was calculated, as her daddy’s money kickstarted her betrothed’s political career in Texas. And that once in power, he would bark orders at her, get jealous if upstaged and not even try to hide his parade of mistresses from her view. Oh, and she really loved purty flowers.

But as Caroli – who has penned several previous tomes on First Ladies – offers, the truth of this marriage is far more complex. And that Lady Bird Johnson was anything but a dowdy, passive victim.

“Lady Bird knew what few others did — that Lyndon trusted her — and only her — with his most important secret — his own frailty,” Caroli writes in the book’s intro. “This big strong man, a genius at politics, could be suddenly undone and once undone had trouble getting himself back on track. When faced with a huge problem or disappointment, he would go to bed and pull the covers over his head, and that’s when she stepped in, to get him on his feet and moving again. Only she could do that.”

Caroli writes as if the couple were destined to be together, both ambitious souls sizing up each other for partner potential. Johnson fell so hard and so fast for the smart, classy but earthbound Bird that he proposed marriage the day after meeting her.

When she demurred (it was rather sort of all of a sudden), he passionately argued his case for matrimony as he would a Congressional bill as they communicated over more than 80 letters, postcards and telegrams during a three-month period when they were separated.

These missives – treasured by Lady Bird and kept in a special box that she would revisit through her life – would form the foundation of their relationship for decades. They only became public on Valentine’s Day 2013, and Caroli is the first major biographer to write about them, saying they contained the “key” to the Johnson marriage.

“It was what he saw that she could do for him, and what she saw that he could do for her. They are romantic and raw and brimming with lust,” she writes. “But they also reveal the implicit deal the pair struck with each other.” That deal was he would do great things that she would be involved with, and she would take care of him, protect him and feed his ego.

Caroli writes that Lady Bird would pull a “psychic veil” over herself when faced with unpleasantness or direct evidence of Johnson’s incessant philandering, which seemed to begin shortly after their wedding date.

Once, she and a Presidential staff member went unannounced to Johnson’s out-of-state hotel room. They found no one there, but the undergarments and lingerie of his secretary were strewn all over the bed in plain sight.

Bird calmly and without comment picked up the clothes to put in a pile, and then went out of her way to be “extra nice” to the secretary. After all, hardly a day went by when she wouldn’t have to see one of her husband’s lovers in some setting. If this unflappable Bird got her feathers ruffled over every one of them, she’d have none left to help her fly.

Why Lady Bird stood for such behavior over the course of decades is perhaps the most perplexing aspect of her personality. Caroli suggests that she viewed it as the dalliances of a powerful and insatiable man – but one who could still not survive without her.

For her part, Caroli says Lady Bird was at times alternately a “fixer, enabler, and smoother-of-feelings” who demonstrated the Chinese philosophy of “leading by pretending to follow.” When her husband insulted someone, she was there to make things right with the aggrieved or his wife. Early on, when he brought constituents, cronies or people he was trying to impress to their apartment at all hours of the day and night, she was there fixing the coffee and a full meal.

She never complained: It was all part of their plan for his – and their – political rise, one in which she was an active participant. Bird once requested a standing, updated list of all births and deaths in Lyndon’s Congressional Tenth District so she could send handwritten congratulations or condolences.

Once the Johnsons' marriage foundation was in place, the stages only got larger as Lyndon ascended to the Senate, the Vice Presidency and the Presidency – Johnson had surprisingly fickle feelings about the Presidency, even as he passed hugely influential laws and grappled with the Vietnam War and his antagonistic relationship with the press.

Through it all, Lady Bird was by his side, grappling with his manic depression, fatalism and narcissism. He could publicly rebuke her so harshly that dinner guests would be forced to stare at their plates, but she was the first person he’d call to act as a surrogate on the stump, critique his press conferences or make an unpleasant call. So devoted was Lady Bird to Lyndon and his every mood and whim that her mothering left a lot to be desired – something acknowledged by both her and their two daughters in later years.

Ultimately, Lady Bird Johnson’s most lasting public legacy would be in the areas of nature preservation, beautification and environmental concerns – passion projects she pursued both through laws and in less formal ways.

And when her husband died in 1973 at the age of 64, she spent decades on these as well as burnishing her husband's legacy until her own passing in 2007 at the age of 94.

But as Caroli reveals, her input into her husband’s political life – and thus the nation’s – ran far deeper than simply looking over the guest list for a state dinner.

And for what it’s worth to the outside observer, despite his faults, Lyndon Johnson held a deep and abiding affection for and sense of gratitude to Bird their entire life together. This book goes a long way into coloring in the spaces about Lady Bird Johnson in what has been a black-and-white line drawing for so many years.

Lady Bird and Lyndon: The Hidden Story of a Marriage That Made a President
By Betty Boyd Caroli
480 pp.
$18
Simon & Schuster