New Jersey said it had lifted the ban on a best-selling book about mass incarceration after the American Civil Liberties Union called for an immediate end to what it said was an “ironic, misguided, and harmful” instance of censorship.

The state’s decision to reverse a prohibition on “The New Jim Crow: Mass Incarceration in the Age of Colorblindness,” by Michelle Alexander, at the two prisons came hours after the A.C.L.U. of New Jersey on Monday sent a letter to the state’s corrections commissioner, Gary M. Lanigan.

The letter argued that the book’s presence on a list of banned texts at New Jersey State Prison and Southern State Correctional Facility violated the First Amendment and the department’s own regulations.

The A.C.L.U. asked that the department confirm that the book had been removed from lists of banned publications and review how its facilities make and carry out choices about banned texts.