The spokesman said the copy of the horror classic will be added to the 19,000-book collection, The Miami Herald reported Monday.
The 1986 novel about a monster lurking in some Maine sewers was the only volume refused from a donation of about 70 new books by a man whose father was killed at the World Trade Center.
Library workers had reviewed the donated books -- which included the works of William Shakespeare, Charles Dickens and Ernest Hemingway -- before accepting all but "It" without explanation, the Herald said.
On Monday, Army Capt. Andi Hahn said the library was wrong to exclude "It" because the novel was approved for circulation and another copy was already in the detention center library.
A contractor identified only as Milton, who has managed the library in recent years, told the Herald he screens books and videos and sometimes excludes those with violent or sexual themes. "It" includes a sex scene between a 12-year-old girl and five boys of about the same age.