Here are my thoughts on this.
These two books were written by men who are very sincere in their heart of what they experienced, but most of what they say is not scriptural.
The Bible says when we are absent from the body we are immediately in the presence of the Lord.
Once an individual leaves this earth and is in the presence of the Lord, Why would they ever come back.
Some of the things that Brother John Piper described in heaven does not line up with scripture.
Now the fellow who went to hell could not have possible gone there because according to scripture their is a great gulf that no one can cross. Secondly, man does not get a second chance.
No doubt these men experienced these things, but was it a vision, a dream, or just something in the subconscious. I believe it to possible all three.
Here are a couple of links that you might concider looking at.
http://www.gotquestions.org/90-minutes-in-heaven.htmlhttp://bromattsblog.wordpress.com/2008/ ... al-heaven/