What comes to this "paradise", well i dont know if its true or not, but there is scientific evidence combined with rationalization that supports the argument. Basically when you die your Na/K channels gets fucked up because you lose the energy to keep to pumps working and to keep them separate, this causes action potentials in both brains and other axons in your body. So basically everything gets activated. This activation of the brains has been observed in mice. Naturally there havent been much study about this on humans, but there has been few times when it has been looked for shortly after death and it has been there obviously, because the nervous system works similarly on all animals. Now if you start to think of the implications of everything getting activated, well the person would obviously lose the sense of time completely, nothing new is "recorded", but what is inside your head gets triggered(i think this is the major cause for why some people with near death experiences has seen something like a film of their life going past their eyes), i think it just feels like a film since nothing is recorded(except maybe in the limited short term working memory). Now when this happens i believe that its not just life moving past your eyes, but since your moral judge in the brains gets activated also(which caused the myth of a judge of afterlife) and all that stuff, the end result can be something that can feel like eternal bliss or hell.
Even tho i dont think it goes exactly like in this book, i think its the closest to actuality, i would highly suggest watching this film:
Now when it comes to karma it could be also what i described earlier, but what is more definite is that if you do bad things to people around you, they will start to act in more negative manner towards you and towards others. This can come back around to you directly or indirectly. Also it has been proven that if you do good deeds(or just even positive thoughts instead of dwelling on negatives), you feel more happy yourself(regardless whether or not you receive good back form others), your immune system works better and im sure it has other good effects on you as well. So this karma thing seems to be real in one way or another.