Why there is two types of books in svenska "Böcker" and "bokar" . What is the difference ?
Bokar is a verb, meaning "to rent/book/reserve." Böcker is the plural noun, "books."
Yup! But it can actually be a noun, too, as in en bok, två bokar - meaning "one beech, two beeches".
Beeches as in the trees? Wait... is this going to turn into another "får får får" sort of thing? Sheep get sheep, beech trees rent beech trees, Buffalo buffalo buffalo Buffalo buffalo...
If you'd like... :) beech beaches beeches beseech beech beaches beeches etc.
(yup, as in trees)
Is there any rule to decide which ending to give words with an article or words that are plural?
I think it would probably be considered a typo of boken, "the book." As far as I know, böken is not itself a real Swedish word.