Question: My first question is what are the scriptures every Muslim (or Islam follower) should abide by? In other words, what scriptures are considered the fundamental/basic on which Islam is based on? Or What are the very important scriptures that are considered as highest/top?

Answer: 
What are the scriptures which every Islam follower should abide by?
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There is only one which is the Quran. There are other scriptures in Islam but they were revealed before the Quran and no longer apply to Muslims. The Tawrat was revealed to mosses as a guidance for him people. All the followers of mosses followed it. After it, Allah (The God) revealed the Injil to Jesus which became obligatory for his followers. 

Few hundreds years after that, Allah revealed the final revelation to Muhammad, the Quran. All the scriptures that were revealed before it (e.g. the Tawrat, the Injil) were abrogated after its revelation. Allah say in the Quran (what means):

> 
> We do not abrogate a verse or cause it to be forgotten except that **We
>  bring forth [one] better than it or similar to it.** Do you not know
>  that Allah is over all things competent?
> 
> 
> 

Let me give an example to clarify how it works:

A student first studies in kindergarten, then grade school, then High school, then collage and then he studies in University. A student can't directly admit into university with no prior education. And a student of University doesn't study the books of schools or collages.

Similarly, the Quran is the highest level of revelation which is revealed for all of humanity. Those who follow the Quran don't need to follow previous scriptures. You might ask, why didn't Allah reveal the Quran to the people of mosses or Jesus?

Cause human civilization didn't reach the level to understand the Quran at that point of time. Just like a school student can't understand or have difficulty understanding University curriculum.
There is only one such scripture in Islam which is the Quran. If someone who claims to be Muslim says he doesn't belief in it then he wouldn't be regarded as one cause all Muslims agree that the anyone who rejects the Quran rejects Islam as well.

All the copies of the Quran are same. They are all written in the same way with the same arabic letters.