As Muslims, we believe in the finality of Prophethood and the finality of the Holy Books. Prophet Muhammad (صلّى الله عليه وآله وسلّم) was the final Prophet. The Quran was the final Holy Book of Allah.
The Shia reject both of these central tenets of Islam. They extend the Prophethood through their Imams, who are considered by them to be superior to Prophets (other than Prophet Muhammad). The Shia also believe in holy books after the Quran, including the Mushaf Fatima (Book of Fatima), Al-Jamia, and Al-Jafr (The Parchment). According to the Shia, these are books divinely inspired from Allah. There are many Shia Hadith which refer to these three holy books * Al-Jamia
The Shia believe that this book, Al-Jamia, contains all the lawful and unlawful as well as all matters that people need. It is considered to be more complete than the Quran, and it is also much larger than the Quran, apparently the size of the leg of a huge camel * Al-Jafr
Another Shia holy book includes Al-Jafr which is a parchment that contains the knowledge of not only Prophet Muhammad (صلّى الله عليه وآله وسلّم), but also of the past Prophets and Infallible Imams. Al-Islam.org says: al-Jafr, on the other hand, is a book which Imam Ali inherited from the Prophet. It contains knowledge of past and future events. Not only this, but it would contain knowledge of the future including the names of every king who would rule on earth. Such knowledge is of course not contained in the Quran, so we see that the Shia belief in such a lofty book really decreases the signfiicance of our Quran. We find the following Shia Hadith in Al-Kafi: How can the Shia then claim to be Muslim when they believe in holy books after the Quran? We do not believe that people of the Bahai faith are Muslim, simply because they follow another holy book after the Quran! Truly, such beliefs constitute Kufr Akbar (Major Disbelief).
Another oddity is that the Shia Imams supposedly possessed books of immense knowledge and yet they hid them from the world, according to the Shia. These Imams did not share these books with their Shia followers. Today, we do not find any Shia who has ever seen these three mysterious holy books. Indeed, the Shia believe that the Hidden Imam has also hidden these books with himself. Imams are meant to guide, so why would they possibly hide books of knowledge? The Prophet (صلّى الله عليه وآله وسلّم) was given the Quran, but he did not hide this holy book from the people. Instead, he shared it with the world and told everyone to spread it so that people be guided.
It would be childish of the Imams to say this book is minestay away! In fact, in the Shia Hadith, one of the Infallible Imams of the Shia says: With us there are such things [i.e. Jafr Al-Jami] that because of which we do not need people [but] instead people need us. It is a bit odd that the Imam would be boasting about this. The only explanation as to why the Shia Imams hid their holy books is that these books simply did not exist and are a figment of the Shia imagination.
The truth is that the fundamentals of the Shia faithin particular the institution of Imamah (which is the major difference between the Ahlus Sunnah and the Shia)are simply not in the Quran. Thus, the Shia are forced to find the tenets of their faith in another book. On the other hand, the rightly guided Ahlus Sunnah will continue to reject any holy books after the Quran, and instead vehemently defends the absolute finality of the Quran and the Islamic message.