So lemme get this straight

[Drich]

Now it is pretty clear that Christianity is a religion of the book, so where in the book does this belief in the trinity come from?

In the verses I alluded to earlier. They establish the trinity by showing God the Father being a separate being, from God the Son, and God the Son being a separate entity from God the Holy Spirit.

Three separate beings with the Same title, forms a trinity. We are told to Worship God, therefore we are to worship the triune God.

And how reliable is it?

It is from the same books that establish Jesus as God the Son.

Whether you consider this reliable or not is not the true issue. In that if One wishes to worship and follow the God of the bible then that person is bound to the bible for revelation. Why? Because the bible sets the tome and understanding for the God described within it's pages. Therefore if one wishes to worship the God of the bible then by the bible is he bound.