What kind of a book is the Quran?

The Holy Quran is a heavenly book revealed for the guidance of mankind. It was sent down on the heart of the Last of Prophets, Muhammad b. Abdullah, peace be upon him and his descendants. The Quran, which is the eternal miracle of the Prophet of Islam (S), was gradually revealed over twenty three years to him. He read out the verses to people and they were attracted to Islam hearing them. The Quran comprises 114 Surahs which consist of 6205 verses and all these verses total 77807 words of which 45653 words were revealed in Mecca and 32154 verses were revealed in Medina. Given the fact that the Quran is the last heavenly book revealed for man's guidance, it is therefore the most perfect book that leads humanity to God.

Detailed Answer
The Holy Quran is a heavenly book revealed by God for prosperity and guidance of humanity to God. It was sent down on the heart of the Last of Prophets and the Seal of Messengers, Muhammad b. Abdullah peace be upon him and his descendants. The Quran is the eternal miracle of the Prophet of Islam (S). This book was gradually revealed over twenty three years to the Prophet (S). He read out the verses to people and they were attracted to Islam hearing them. The Quran comprises 114 Surahs (chapters) which consist of 6205 verses and all these verses total 77807 words of which 45653 words were revealed in Mecca and 32154 verses were revealed in Medina[1]. Given the fact that the Quran is the last heavenly book revealed for man's guidance, it is therefore the most perfect book that leads humanity to God.
In order to give a proper description of the Quran, we must turn to the Quran itself. When we refer to the Quran, we see that this great book mentions a number of attributes and characteristics for itself, which reveal part of the truth about it. Some of the qualities mentioned are as under:
1. The Quran says:
"ذلك الكتاب لا ریب فیه هدی للمتقین"[2]
This is the Book; in it is guidance sure, without doubt, to those who fear Allah.
In this holy verse, the Quran introduces itself as a book in which there is no ambiguity or vague points. If we look at historical proofs and evidences, we will come to know that apart from the Quran which speaks so clearly, there is no other to make such a claim. Therefore, the first attribute of the Quran is that there is no dark or inconspicuous point in the Quran. The Quran is a clear proof.
2. Then it says, "Guidance to those who fear Allah". Those who are God-fearing and protect themselves against evil are the ones who are guided.
3. Elsewhere the Quran says:
"هذا بصائر من ربكم و هدی و رحمة لقوم یوقنون"[3]
These are clear proofs from your Lord and a guidance and a mercy for a people who believe.
4. In another verse, the Quran says:
"هذا بصائر للنّاس و هدی و رحمة لقوم یوقنون"
These are clear evidences to men and a Guidance and Mercy to those of assured Faith.
5. The Quran says in another verse:
"... قد جاءكم مّن الله نور و كتاب مّبین، یهدی به الله من اتّبع رضوانه سبل السّلام و یخرجهم مّن الظّلمات الی النّور باذنه و یهدیهم الی صراط مّستقیم"[4]
"Indeed Our Messenger has come to you making clear to you much of what you concealed of the Book and passing over much; indeed, there has come to you light and a clear Book from Allah; With it Allah guides him who will follow His pleasure into the ways of safety and brings them out of utter darkness into light by His will and guides them to the right path."
There are points in these verses which give us a clear description of the Quran:
A) This book is a book of guidance, blessing, insight and illumination.
B) Whoever turns for guidance to this book in which there is no ambiguity and uncertainty, will be guided.
C) The believers, and those who are firm in their hearts enjoy an important position with the Quran.
The Messenger of Allah, peace be upon him and his descendants, has formally challenged others saying that the Quran is not his work and that he did not scribe it; it is the word of Allah. He says that no man can produce the like of it and if you do not believe you can test it and seek help from whomsoever he wants but he must know that even if jinnis and humans unite with each to produce the like of it, they will not be able to do so:
"قُلْ لَئِنِ اجْتَمَعَتِ الْإِنْسُ وَ الْجِنُّ عَلى أَنْ یَأْتُوا بِمِثْلِ هذَا الْقُرْآنِ لا یَأْتُونَ بِمِثْلِهِ وَ لَوْ کانَ بَعْضُهُمْ لِبَعْضٍ ظَهِیراً"[5]
Say: "If the whole of mankind and Jinns were to gather together to produce the like of this Qur'an, they could not produce the like thereof, even if they backed up each other with help and support."
This verse calls in the most clear terms on all people of the world, young and old, Arabs and non-Arabs, human beings and even sane beings other than humans, as well as philosophers, scholars, literary men and women, historians, geniuses etc. without any exception to confront the Quran. It says, if you think that the Quran is not the word of God but the product of human thought, then you are also human beings and you should also bring forth the like of it. This invitation on the part of the Quran which is technically called "tahaddi" (Challenge) is one of the basic elements of every miraculous deed. Wherever there is such a phrase used, we clearly understand that this is a subject of miracle.[6] The Quran brought forth extensive notions which later on became a source of inspiration for philosophers and scholars of law, jurisprudence, ethics, history and so forth. The Quran has the most beautiful sayings about mankind. The Quran has endorsed Torah and Gospel but it has also said that these books have been tampered with and then man's treacherous hands have changed them. The Quran corrects the mistakes of these two books in regards to metaphysics and stories of prophets as well as certain rules. The Quran announces that God, the Glorified, is pure from all the allegations about Him such as wrestling and the likes which have wrongly ascribed to him. Also, it purifies the prophets of all false attributions which have been mentioned in the previous books. This is by itself another proof of the rightfulness of this book.[7]
Muslims from the early period of Islam as of now have been attaching great importance to Quran. This is indeed an indication of their love and inclination towards the Quran. In the time of the Holy Prophet (S), the Quran was scribed by a number of people who were designated by the Prophet (S) and were known as "scribe of the revelation". In addition, most of Muslims, men and women, young and old, had a great passion for memorizing all or a part of the Quran. They recited the Quran in their prayers and believed that its recitation earned them sawab (reward) even in ordinary situations when they were not busy offering prayers. They enjoyed reciting it as it was a source of peace and tranquility for them. Muslims' love and passion for the Quran led to the coming into being of some literary and rational sciences which, in the absence of the Quran, would not come into being.
The Quran is also unique in its style. The Quran's style is neither poetic nor is it in the form of a prose. It is not poetic because its verses lack any rhythm and rhyme. The Quran's style and method has no precedence and antecedence. That is to say, no one has written or spoken in a Quranic style in the past nor has anyone later been able to imitate it or produce the like of its verses despite the Quran challenging all human beings.
In a nutshell, owing to their love and passion for the Holy Quran, Muslims in every era have worked about this heavenly book proportionate to their intellectual and practical means and capacities. They have, for instance, tried to learn, memorize, recite, interpret and explain its verses and words in whatever manner possible. They have even given meticulous attention to the number of verses, words and even letters in the entire Book using them in legal, moral, social, philosophical, mystical and scientific issues.[8]
For further information about the Quran, kindly read the following indexes:
The Miracles of the Quran, 70 (site: 311)
Definition of a Miracle and Method of Proving it, 115 (site: 985)
The Quran is a Revealed Book, 67 (site: 308)
Descent of the Quran, 71 (site: 314)
Compilation of the Quran by the Messenger of Allah, 1625 (site: 1632)
Conditions for understanding verses of the Quran, 193 (site: 2511)
Quran, the Non-Distorted Book, 3213 (site: 3938)
The Quran's Challenge to Others, 4674 (site: 4942)

[1] See: Website of Thematic Encyclopedia of the Quran affiliated to Quran Culture and Sciences Center
[2] Baqarah, 1
[3] Al-A'raf, 203.
[4] Al-Maedah, 15 and 16
[5] Al-Israa, 88.
[6] Makarem Shirazi, Naser, Tafsir Namunah, vol.2, p. 274, Dar al-Kutub al-Islamiyah, Tehran, first edition, 1374.
[7] Mutahhari, Murteza, Collection of Works, vol.2, p. 212 - 213, Sadra Publications, Tehran, fourth edition, 1380.
[8] Ibid.