There are many Muslims believing such holy texts, but it is not so right.
According to Islam, we believe that the Qur'an isn't corrupted, it is just as exactly it is announced by Prophet Mohammad(puh). (There is a claim about recm verses, but this claim isn't approved.) We also support this belief with historical facts. Qur'an is a word of Allah, thus it is the most sacred word and we have to obey it.
There are hadiths (Prophet's words and actions), which are collected in various books and are separated from Qur'an. Hadiths are considered as 3 groups:
- Genuine ones: They are also sacred, but not as sacred as Qur'an. We have to obey them too.
- Non-genuine ones: They are made up, they have no historical support, they have contradictions with Qur'an. They certainly aren't sacred, we have to avoid them.
- Controversial ones: We have to be very careful about them and leave the decide to determine whether they are genuine to the scholars.
We know that by Surat An-Najm 53/1-4, Mohammad(puh) didn't act against Allah. But, his actions and words were told by his nearby and collected later from his death. Some hadiths are proven that they are made up. Made up ones are not sacred of course, but his true words and actions are sacred, because they are the explanations of Qur'an and their practices. Some hadiths are genuine, but relevant to his time and conditions. For example the amount of zakat. This is why hadiths are not as sacred as Qur'an. Qur'an's rules and limits are for all time and conditions, they can't be adjusted.
There are also tafseer (explanation of Qur'an) books written by scholars. They are comments and conclusions of their authors, sometimes contradicting each other. Many Muslims behave that as they are sacred, but I think they are not. Because they are not actual words, but comments and conclusions, they may be wrong. Scholars are humans and can make mistakes.