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: 
All perfect praise be to Allah
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When you say **scriptures**, than let us be all clear, that **Quran is the word of Allah, sent down on prophet Muhammad ﷺ by Jibrael A.s. And as Muslims this is the most important book we have to follow, indeed this book is basis and Allah promise will not be corrupted until the day of judgement!!**

Than in Quran, you will find **Allah's command on following prophet** in multiple places, **how can we follow prophet in present time? we will look for his sayings, actions etc. Those are saved in books of ahadees.** There is complete SCIENCE of ahadees and how they are judged to be authentic etc. In short, we can trust all authentic ahadees. So, ahadees should be followed, they contain all the sayings, doings, routines etc of prophet.

Now, **ahadees are a large collection**. As one can expect that everything(his saying/doing/routines etc) in a person's life is written, it will be a huge stack... So are ahadees. And every Muslim can't find the thing he is looking for in ahadees easily(because of their volume). So, he will need to follow someone he trust. You may call it taqleed or whatever. But, he must leave anything which he finds is contradicting with something in Quran and hadees, and after talking about it and discussing it, his heart is telling that he(whom he is following) is wrong in this particular matter and some other view is right about this. Than one must follow the view(he thinks is right).

**So, as of scriptures(as the question ask about) these 2 are main scriptures from where each islamic teaching is derived/taken from.**

I hope this answers the question, although i am not sure what you are actually looking for but i answered what i understood in question.

Allah knows best
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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.