Valley of the Dolls by Jacqueline Susann (1966)

From Goodreads:

At a time when women were destined to become housewives, Jacqueline Susann let us dream. Anne, Neely, and Jennifer are ambitious young women in New York City who become best friends as they struggle to make their mark. Eventually, they climb their way to the top of the entertainment industry only to find that there's no place left to go but down, into the Valley of the Dolls.

Five decades after it was first published in 1966, Valley of the Dolls remains the all-time pop culture classic, a pioneering work that tackled women's rights, gay rights, drug addiction, and the male literary establishment with signature flair, going on to become one of the world's most popular novels and an influence on generations of cultural icons. Power, money, celebrity, and the deadly seduction of prescription pills — fifty years on, a bedtime dose of Valley of the Dolls is just as indispensable, and just as gloriously, scandalously fun, as ever.