IF there is a spirit world, and we are spirits in the flesh for a short time, then what is the purpose of our lives?

We wanted a God-Body so we would no longer be Bored Cherubs ; but now we have to become perfect , or else we punish ourselves ( subconsciously ) .

Well we are born as baby spirits, innocent. We start off by trusting or not trusting. As we grow we learn grow experience everything. " For God gave us a time for everything". Mainly we are to grow, learn, gain wisdom. Knowledge brings wisdom. Secular measures in success. Spirit in wisdom, brings power. Jesus said " What's it profit a man to gain the world but lose soul?"

The Bible gives us an example Romans chapter 1 verse 18 through 27 tells us that the invisible of God is clearly made seen by the things that he has made even his eternal power and godhood so that men are without excuse.

Take a look at the worm that dieth not!

A caterpillar is Born Into the world and stuffs its face with food night and day and has no idea that one day that it will become a butterfly and toils day and night to eat and eat and eat and birds and other animals feast on caterpillars.

One day the caterpillar decides it's time for me to spend my cocoon it climbs in that cocoon with no fear of that cocoon and no knowledge that it's going to ever come out of it it just does it but when it comes out of its cocoon it's a butterfly behold the worm which dieth not!

In case you haven't figured it out we humans are that worm! Like the caterpillar that feasts on green leaves and has no idea what's really going to happen way to change when we die

You were created to be a warrior for God with the ability to defect if you wish. You are daily choosing your eternal destiny of with God or with Satan forever.

We do not have the capacity to understand or see this spirit world or prove it.

For those eternal spirits who have rejected God, their end is the pits of eternal hell in conscious torment.

For those eternal spirits who have received forgiveness of sins through Jesus Christ, they will go to heaven.

As far as what they will do:

1 Corinthians 2:9

But as it is written: “Eye has not seen, nor ear heard, Nor have entered into the heart of man The things which God has prepared for those who love Him.”

In this case, "If" is a hell of a big word.

In my opinion and experience: to grow, learn and develop through our life events and relationships here. For oneness/divinity to get to experience itself through apparent opposites. And maybe also just because it's such a great adventure... (As in: why the hell NOT?!) ;-)

I haven't read the book you describe, though. And I'm not a Christian. But I'm a spiritual person in my own way.

No one has demonstrated that spirits and the spirit world is real. So, it is not real; it is fake.