Kwamenat, your question of the doctrine of an eternal hell is a fair question. The book you have linked by some person, Joe Crews, from helltruth.com has some errors in it.
I do not intend to make a full rebuttal but I will make a quick remark. The Bible promises everlasting torment, suffering, and punishment.
Matthew 25:46. "And these shall go away into everlasting punishment: but the righteous into life eternal."
The author of this book, Hell-Fire, makes a silly mistake on this point. He assumes that because "eternal life" is not promised to the damned that therefore this equates to "eternal death." Now, eternal death is an expression never used in the Bible; the very idea is non-existent. We do hear about "spiritual death" but that is evidently not Annihilationism.
Very simply, eternal life is not used in the Scriptures primarily as a quantitative expression of life, but as a qualitative expression. Eternal life is referring to the abundant life of joy and peace, etc., in Christ. Spiritual death, or eternal punishment, is the deficient life of those who are by nature enemies of God.
Moreover, I do not consider "hell-fire" to be a literal fire. I perceive hell-fire to be a profound description of how sinners experience the overwhelming Holiness of the Presence of God. (Deuteronomy 4:24.)
Nahum 1:6. "Who can stand before His indignation? and who can abide in the fierceness of His anger? His fury is poured out like fire, and the rocks are thrown down by him."
Isaiah 33:14. "The sinners in Zion are afraid; fearfulness hath surprised the hypocrites. Who among us shall dwell with the devouring fire? who among us shall dwell with everlasting burnings?"
Some of the best teaching which I personally enjoy on hell are by a gentleman, R. A. Finlayson, who says,
"Hell is eternity in the presence of God without a mediator. Heaven is eternity in the presence of God, with a mediator."
And,
"Hell is an encounter with the character and will of God. God's nature is holy, and His character is unsullied righteousness. For the unholy to mix with holiness is Hell."
I would recommend reading the following two articles:
The Holiness of God by R. A. Finlayson
The Last Things [Judgement] by R. A. Finlayson