When people ask me about my device, I try to be less pretentious and condescending towards them.
So let me redress the original poster's questions with my own answers, however witless:
Why don't you just carry a book?
Because I care enough about portability that I'm usually willing to trade quality and tangibility for it.
Why is the screen not color?
Because the technology is not yet sophisticated enough to display color.
Can i watch videos on it?
Unfortunately the screen technology is too primitive to support such things.
Why does it not have unlimited memory?
Because it's essentially a crude tablet computer, and all computers have finite memory.
Can i get [religious book] on it?
You would have to search for it, and perhaps download it illegally, which may be offensive in your belief system, but there's a good chance you could find it. For a biblical scholar or collector though, it's always best to have dedicated paper copies around.
Do i have to download books for it?
Yep, unless you want to stick with the free bundled books that come with the device when you buy it.
Does it have an inbuilt printer?
As amusing as that would be (and useful for document readers), it's not really feasible now.