Originally Posted by FinalFantasyFan
The English language is read from left to right. Thus, the flow of the language also goes from left to right. This means that written material proceeds from left to right.
Japanese characters are read from right to left, dictating the flow of language on a page and thus the direction of a book.
Publishers change the "direction" of a manga in order for the flow of the book to make sense to people who have been taught from the first days of school to read from left to right. It provides less of a shock for the casual bookstore patron.
The current trend for North American publishers to start printing manga in the "Japanese" format has two sources:
1) It's cheaper to print the book since they can eliminate the step of having to transpose all the pictures to their mirror image.
2) Marketing research has shown that manga printed in the "American" style is instanstly reviled by the japan-centric consumers for not being "correct" and sales start to lag.