I've seen this movie multiple times throughout the years and recall thinking it quite good. It's been at least a few years since the last time I saw it, and I no longer am able to remember the specifics at this point.

Just now I checked Wikipedia, to see what plot details are mentioned there. Anyone not wanting to learn spoilers, don't read beyond this point. Everyone else, here's what Wikipedia says:

After a military officer (Bruce Cabot) gets Ann Vickers (Irene Dunne) pregnant and leaves her, she and friend Malvina Wormser (Edna May Oliver) go to Havana, where she gives birth, though the child dies soon after. Feeling conflicted and regretful, Ann devotes herself to social work, taking a job in a women's prison. However, when she tries to improve the conditions there, she loses her job. She instead writes a book about the harsh realities of the prison and begins a romance with a married judge, Barney Dolphin (Walter Huston). Though progressive in his views, Dolphin is caught taking bribes and is sentenced to prison. Ann, once again pregnant, supports herself by writing until Dolphin is released a few years later. Finally, Ann, Dolphin, and their child are reunited.

Meanwhile, here's what TCM.com says:

After she is seduced and then abandoned by the handsome but fickle World War I army officer Captain Resnick, Ann Vickers, an ambitious, devoted social worker, travels to Havana with her friend, Dr. Malvina Wormser, to have her illegitimate baby in secret. When the baby dies, however, Ann, deeply embittered by her romantic experiences, rejects the marriage proposal of longtime companion Lindsey Atwell and rededicates herself to her career, landing a job as a sociologist at a women's prison. Soon horrified by what she observes there, Ann protests conditions to the prison authorities. Instead of correcting matters, Ann's superiors frame her in a phony "sex" setup and threatened her with blackmail. Forced out of the job, Ann retaliates by writing a best-selling book about the terrors of prison life, which, with the help of influential Judge Barney Dolphin, earns her a post as head of another women's reformatory. Although the job challenges and enriches her, Ann still longs for romance and children, and upon at last meeting the successful but unhappily married Barney, she falls in love. Their romance blossoms and Ann becomes pregnant again, but as Barney is unable to secure a divorce, she must bear their son Matthew still unwed. At the same time, Barney is indicted and found guilty of accepting bribes and is sentenced to six years of hard labor. Because of Matthew's illegitimacy, Ann is made to resign her position at the reformatory but is content just to write free-lance articles to support Matthew and wait for Barney's release. To her joy, Barney receives a pardon after three years and, now divorced, returns home to marry Ann, who tells him that she is cured of her ambition and is happy to be a simple homemaker.

Has anyone ever read the book?

I haven't.