I think one book that changed a lot was the Qur'an - it made me an atheist. I don't come from a muslim background but it made me realize, how some religions (well at least the abrahamitic ones) came to be.
It matters a lot since I've seen eastern books having hugely varying translations.
For example, an Indian friend of mine tells me that 'I am become God ... ' is a very inaccurate representation of the original hindi text