Often you will see many books speculate on Christianity. You see this particularly in areas which the bible has little interest in speaking.
Some of the most common areas for speculation are heaven, angels, and demons. The bible certainly discusses all of these themes but people often push well past the information in the bible.
And the issue is that speculation past what the bible says is just speculation. Not only that it is not neutral but negative.
Let me explain. You see one reason God does not tell us everything is so we can be humble. It is an exercise of human pride to try to comprehend spiritual things that God does not reveal.
In many aspects of life man can discover things for himself. But many spiritual things can only be revealed. Man needs to have the humility to accept what is revealed and accept that he does not have all the answers he may wish to have all the time.
Man always wants to know more and God knows what man needs to know. God has not withheld anything of use for us, but we like to think he has.
And in a way much speculation on areas in the bible is a repeat of man's first sin. Man wishes to know what God does not wish to tell him.
We need to trust God that he has revealed the spiritual things we need and accept that what he has revealed is truthful even though he has not revealed everything we could always wish to know.