The Assumption of Mary is considered a revealed truth apart from Scripture. Roman Catholics do have some passages which allude to an ascension but they aren't very clear.

Where did you find that the Bible isn't the best selling book of all time? I'm looking for sites on it but can't find anything. I've read in some books that the Bible is best selling all time and has sold in the billions.

Guiness book of records isn't just one book anyways, they keep revising it. The Bible has stayed the same the whole time so this is 2 different types of books.