I'm involved in introducing Islam to non-Muslims. I would like you ,please, to tell me the best way to explain the reason behind different ways of reciting the Holy Qur'an(Qir'aat). I believe the Qur'an is one and that it has been preserved by God and it cannot be tampered with. However, I'd like to know the best way to explain this to non-Muslims, especially to those who have recievd wrong information from orientalists etc.. I know that ,mostly, the differences are in the way of pronounciation but there are differences in words ,sometimes. For example, in chapter 101, verse 5 it reads ,"Kal Ehni" and in another Qir'aat it reads, "Kassofi". There is another word in another verse which reads ,"Fatabayano" and in another Qir'aat it reads,"Fatathab-batoo". Can I have detailed information on the ways these Qir'aats reached us and the reason behind the differences behind them? I would appreciate giving me refernces from the Qur'an and the Hadith. Also, if there is a god refernce about this subject in Arabic and English, I would appreciate giving me its name. It would help if you know of an Internet site which explains this subjest. I know that these are many questions, but I need this information disperately and may God reward you. Amen!

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The reason for the differences in the way of reciting the Qur'an is that the Qur'an was revealed to the Prophet in seven different dialects as it is narrated through many sound chains. It is narrated in Sahih Bukhari and Muslim from Ibn Abbas that the Prophet said: "Gabriel has recited the Qur'an to me in one way; then I requested him (to read it in another way), and continued asking him to recite it in other ways, and he recited it in several ways till he ultimately recited it in seven different ways". Imam Muslim narrated from Ibn Shihab that he said those seven different dialects are in recitation only and they do not have any effect on the meaning, i.e. the Halal and Haram.
The Prophet said: "The Qur'an has been revealed in seven different ways, so recite it in the way that is easier for you". It is a fact the difference in the ways of recitation of the Qur'an is a difference of varieties not a contradiction between the various dialects. Any contradiction between the words of Allah (The Qur'an) is impossible. Allah Says (interpretation of meaning): {Do they not then consider the Qur'ân carefully? Had it been from other than Allâh, they would surely have found therein much contradictions.} [4: 82].
Imam Ibn al-Jazari said: "We consider the recital ways of the Qur'an we find according to three conditions.
First, The difference of words but the meaning is the same such as:
عَلَيْهُمْ - عَلَيْهِمْ (Alaihim, Alaihum) / اَلْقُدْس - اَلْقُدُس (al-Qudus, al-Quds) /
يَحْسُبُ - يَحْسِبُ (yahsibu, yahsubu).
Second, The difference in both, i.e. in the words as well as the meaning but in the last they indicate only one object; for example. مَلِكُ (Malik i.e. King) مَالِكُ (Malik i.e. Lord) in chapter al-Fatiha verse 3. Both words, though they are different in pronunciation and meaning, they indicate only one thing. Here they mean Allah the Alone. Another example is from chapter 2 verse 259 نُنْشِزُهَا (Nunshizuha, i.e. we bring them together till they become complete) نُنْشِرُهَا(Nunshiruha i.e. we animate them). Thus Allah put two meanings in two different ways of recitation.
Third, the difference in both, i.e. in the words and meanings though they indicate various objects, there is no contradiction between them. For example, see the verse 110 in chapter 12 ( (وظنّوا أنّهم قد كُذِّبُوا (Kuzziboo, i.e. The Messengers believe that their people have accused them of lying.)
In another way of recitation (وظنّوا أنّهم قد كُذِبُوا) (Kuziboo, i.e. The Messengers come to think that they were treated as liars).
It becomes clear from the above details that there is no real contradiction in meaning among the ways of recitation; the difference appears superficially only, whereas there is no difference in the essential matters.
As for the benefits of the differences among the ways of recitation, they are numerous. Among them are:
1. To make the recitation of the Qur'an easy and convenient for Ummah.
2. To prove the miracle of the Qur'an with complete abridgement.
3. To prove with clear signs that the Qur'an is revealed from Allah because, despite the mentioned differences they support each other in a fabulous and attractive style. This coordination proves that the Qur'an is a miracle and the Prophet and all his teachings are true. Allah Says (interpretation of meaning): {Do they not then consider the Qur'ân carefully? Had it been from other than Allâh, they would surely have found therein much contradictions.} [4: 82].

4. It is clear that Allah has reserved the protection of His words (Qur'an) for Himself. This is yet another sign of the authenticity of the Qur'an, that makes knowledge of His Religion in all places and times through authentic scholars and others who memorize the Qur'an, it words, know its different recital ways, clarify its explanation, meanings and differentiate between the correct and authentic narrations and those which are incorrect and weak. They point out the more appropriate recitation of the Othmani Mus-haf (the authentic one copied from the original text of the Qur'an written in the period of the Prophet) from those which are not so.
Dear questioner brother,
The examples you have mentioned in your question are similar to the examples we have mentioned before. The word "Fatabayyanoo/ Fatashabbatoo" Imams Hamza, Kasae and Khalaf recite "Fatasabbatoo", while others recite it "Fatabayyanoo". Both ways of recitation are authentic and they are in concordance with the Othmani Mus-haf, since the Osmani Mus-haf does not have the of the alphabets, accents that differentiate words that are similar in writing.
As for the word "Ka al-ssoof" in chapter 101, as you mentioned in your question, it is not proven in an authentic way of recitation because it contradicts the rules and regulations of the correct ways of recitation.
As for references in this matter you may refer to
1- النشر في القراءات العشر لابن الجزري
2- غيث النفع في القراءات السبع
3- مناهل العرفان في علوم القرآن عبد العظيم الزرقاني.
Allah knows best.