What is the only book in the Bible where God is not mentioned?

The book of Esther

I've also heard it mentioned that God isn't mentioned in the Song of Solomon.

And they cried to God: and as they were crying, a little fountain grew into a very great river, and abounded into many waters. - Esther 11, 10

And when Mardochai had seen this, and arose out of his bed, he was thinking what God would do: and he kept it fixed in his mind, desirous to know what the dream should signify. Esther 11, 12

Thou art Lord of all, and there is none that can resist thy majesty. - Esther 13, 11

But I feared lest I should transfer the honour of my God to a man, and lest I should adore any one except my God.

And now, O Lord, O king, O God of Abraham, have mercy on thy people, because our enemies resolve to destroy us, and extinguish thy inheritance. - Esther 13, 14-15

There are other examples that I didn't post.
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Although the book never mentions God by name, an allegorical interpretation justified its inclusion in the Biblical canon. - Wikipedia; Song of Solomon

I did not check the Song of Solomom; I am taking Wikipedia's word. I have no idea if any other books do not mention God, in any other His names. Perhaps it is that the name of Jehovah is not mentioned.
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I slso found this

It could have been interesting to compare 1Macc's attitude towards the divinity to the book of Esther, where God is not at all mentioned.

on this site.
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Esther is the only book in the Bible that does not contain the word "God."

But later, on the same page, is this:

The word "God" appears in every book except Esther and Song of Solomon.

This discrepancy is on the page owned by the Manistique Bible Church ©.
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I'm going to stop looking now.