Contrary to the verses which consider guidance of the Quran particular to the pious, there are other verses in the Holy Quran which regard the divine books and the Quran as a source of guidance, remembrance (zikr) and warning to all people especially the pious. The Quran says about common guidance: "Ramadhan is the (month) in which was sent down the Qur'an, as a guide to mankind, also clear (Signs) for guidance and judgment (Between right and wrong)." It is inferred from this and similar verses that the guidance of the divine books, especially the Holy Quran, has been mentioned in two ways: One, guidance and awakening for the people of the world; two, guidance for the pious.
When it comes to the guidance of the pious, it has been said that there are two guidance, one before they became pious, the other after it. The first guidance made them pious; and thereupon Allah raised their status by the guidance of His Book. Thus, the first guidance is in need of man's healthy and unimpaired nature and the second guidance is by the Quran and subordinate to the first guidance. The circle problem raised in the question is solved with this explanation.
There are different verses in the Holy Quran which consider the guiding aspect of this holy Book exclusive to the pious giving rise to the question that if the Quran is the book of guidance, what is the use of guiding those who are already guided? The Quran can be a book of guidance only when it also guides the impious.
In response to the above question, it is necessary to take notice of the following points so that it may be possible to come to a conclusion at the end:
A) There are verses in the Quran which generalize guidance for all people. These verses consider the divine books and the Quran as a source of guidance, remembrance and warning to all people not only the pious ones. God, the Almighty, says:
"It is He Who sent down to thee (step by step), in truth, the Book, confirming what went before it; and He sent down the Law (of Moses) and the Gospel (of Jesus) before this, as a guide to mankind, and He sent down the criterion (of judgment between right and wrong). Then those who reject Faith in the Signs of Allah will suffer the severest penalty, and Allah is Exalted in Might, Lord of Retribution."
Else where, the Quran speaks of common guidance: "Ramadhan is the (month) in which was sent down the Qur'an, as a guide to mankind, also clear (Signs) for guidance and judgment (Between right and wrong)."
About the Quran being a reminder, it says: "It is naught but a reminder for the nations." And about the Quran being a warner, God says: "Blessed is He Who sent down the Furqan upon His servant that he may be a warner to the nations."
It is inferred from such verses that the guidance of the divine books especially the Holy Quran has been mentioned in two ways: One, guidance and awakening for the people of the world; two, guidance for the believers. That is to say, the Quran is a book which guides people and whoever turns to it without any evil intention will be guided in the right path.
This point is evidenced by Quranic verses which consider the Quran as a guide for every individual who has decided to tread on the right path, or it can be said that the Quran has been sent to mankind so that those who possess an unimpaired nature may benefit from it and it is also a warning to the oppressors and tyrants.
Piety, on the other hand, is a comprehensive virtue that runs through all the ranks of the true faith. It is for this reason that Allah has not reserved this adjective for any particular group of the believers.
B) The Existence of two kinds of guidance for the pious believers in Quranic teachings:
As it is inferred from the Quranic verses, the disbeliever and the hypocrites are surrounded by two straying and two blindness. Their first straying causes their unbelief and hypocrisy, and the second one (which comes after their unbelief and hypocrisy) confirms their first error and strengthens it. Look at what Allah says about the disbeliever: "Allah has set a seal upon their hearts and upon their hearing; and there is a covering over their eyes." Sealing their hearts has been ascribed to Allah, but the covering over their eyes was put by the disbeliever themselves. Likewise, Allah says about the hypocrites: There is a disease in their hearts, so Allah added to their disease. The first disease is attributed to the hypocrites themselves, and the second one to Allah. The same reality has been explained in many verses. For example: He causes many to err by it and many He leads aright by it! But He does not cause to err by it (any) except the transgressors; ...but when they turned aside, Allah made their hearts turn aside.
If a man's nature is healthy, it cannot fail to see that it is dependent on some thing above it. The Quran says in the second verse of Sura Al-Baqarah, "This Book, there is no doubt in it, is a guide to those who guard (against evil)." The characteristics of piety, enumerated in these four verses, are five: Believing in the unseen, keeping up prayers, spending benevolently out of what Allah has given, believing in what Allah has revealed to His apostles, and being sure of the hereafter.
Allamah Tababtabai (r.a.) says in rejection of the vicious circle which may see to some people as such: The pious ones acquire these spiritual qualities by a guidance from Allah, as Allah tells us in the next verse: "These are on a guidance from their Lord." They became pious and guarded themselves against evil because Allah had guided them to it. When they got that quality, the Qur'an became a guidance for them: "This Book ...(is) a guidance to those who guard against evil."
It clearly shows that there are two guidance, one before they became pious, the other after it. The first guidance made them pious; and thereupon Allah raised their status by the guidance of His Book.
In other words, the first guidance must have been before the Qur'an. They must have been guided by a healthy and unimpaired psychology. If a man's nature is faultless and flawless, it cannot fail to see that it is dependent on some thing above it. Also, it realizes that every other thing, which it may perceive, imagine or understand, depends likewise on a thing outside the chain of dependent and needy things.
Thus, it comes to believe that there must be a Being, unseen and un-perceptible through the senses, who is the beginning and end of every other thing. It also sees that the said Essential Being does not neglect even the smallest detail when it comes to creative perfection of His creatures. This makes him realize that the said Creator cannot leave the man to wander aimlessly hither and thither in his life; that He must have provided for him a guidance to lead him aright in his actions and morals. By this healthy reasoning, the man acquires the belief in One God, in the institution of prophethood and in the Day of Resurrection. In this way, his faith in the fundamentals of religion becomes complete. That faith leads him to show his servitude before his Lord, and to use all that is in his power - wealth, prestige, knowledge, power, and any other excellence - to keep this faith alive and to convey it to others. Thus we come to the prayer and benevolent spending. The five virtues enumerated in these verses are such that a healthy nature unfailingly leads the man to them. Once a man reaches this stage, Allah bestows on him His another grace, that is, the guidance by the Qur'an. The above-mentioned five qualities - correct belief and correct deeds - fall between two guidance, a preceding one and a following one. This second guidance is based on the first one.  As a result, there is no vicious circle here.
 - Of course, in some of the verses of the Holy Quran, certain holy Books such as the Gospel has been considered as an exhortation and guidance for the believers. God says in Sura Al-Maedah verse 46: "And in their footsteps We sent Jesus the son of Mary, confirming the Law that had come before him: We sent him the Gospel: therein was guidance and light, and confirmation of the Law that had come before him: a guidance and an admonition to those who fear Allah. "
 - Al-Baqarah, 2; Aal-e Imran, 138; Al-Noor, 34, Al-Nahl, 89; Al-Nahl, 102; Fussilat, 44; Yunus,57.
 - Aal-e Imran, 3 & 4.
 - Al-Baqarah, 185
 - Takwir, 27 – 28, "It is naught but a reminder for the nations."
 - Ya-Seen, 70, "That it may give admonition to any (who are) alive, and that the charge may be proved against those who reject (Truth)."
 - Al-Baqarah, 7
 - Al-Baqaraj,10
 - Al-Baqaraj, 26
 - As-Saff, 5
 - Tabatabai, Muhammad Hussein, translation of Tafsir Al-Mizan, vol.1, pg. 70, Islamic Publications Office, Qom, 1374 (1995).
 - Ibid, vol.1, pg. 71