Disbelieving the Qur'an is irrational
Firstly what is the Qur'an? The word Qur'an means a recitation or a proclamation. It claims to be the exact word of God as revealed to Muhammad. These words form an essential part of the prayers of Muslims and reading the Qur'an for Muslims is often a matter of recalling directly from memory rather than reading the text from the book. It is essential to keep this in mind when reading the book. It is not a text book to be read once but a book to be returned to time and time again for moral guidance and inspiration.
To disbelieve in the Qur'an is to deny that it is revelation sent by Allah to Mankind. To find out why no other explanation is rational read the section on the Amazing Qur'an of the online book called Islam for the Western Mind.