Can a book be written without an antagonist?
Yes, it can.
I'm answering late and have read the other answers. I had to look it up, but in every dictionary reading I have found, "Antagonist" is a person or "One who opposes ...", and in this context I think "one" is obviously singular, and refers to a person with intent to oppose (or in scifi or fantasy, a sentient being capable of such intent).
I don't think "antagonist" and "conflict" are synonymous, if anything it is closer to "antagonist" and "villain" being synonymous.
A book can be written without any specific villain or villains. A story like the Tom Hanks movie Cast Away, marooned alone, needs no specific villain with intent. The pain is thirst, hunger, heat, isolation, medical emergency, the triumph is finding solutions to those problems. No villain with intent is required.
Another plot could b a woman coming to terms with her imminent death due to cancer. No villain with intent is required.
Or, a teen girl struggling with the realization she is gay. This needs no villain with intent, it can be her struggle with her own beliefs and expectations, her anticipations about how her parents, siblings and society in general will treat her. She simultaneously wants to be a conformist and fit in, and desires a non-conformist love life and sex life.
A book cannot be written without conflict of some sort; a disparity between what exists and what is desired, puzzles to be solved, physical or emotional pain, hardships. Irrevocable events, like a death, that demand adaption to a new reality.