I apologize if this question has been asked already (did a quick search and didn't see anything).
Since the idea of scripture validating itself seems circular, how does one validate the 66 books of the Bible without appealing to some kind of subjective source outside of scripture? It would seem to me that the three ways are:
1) God reveals it to you directly
2) internal witness - bosom burning(subject to error)
3) external witness - experts,pastors(subject to error)
How to resolve this dilemma?
Frankly, all systems of knowing are self-referential: empiricism, rationalism, and biblicism all suffer from this critique. I can't see how a quick search didn't turn up anything as there are pages and pages of websites dedicated to just this question. Let me help you with a clearer search, as this forum is too small to consider such a large question. Look for "reformed epistemology" - epistemology is the philosophical science of knowing how we know what we know.
For confessional Presbyterians, the answer is given a solid head start on thinking through the matter via the Westminster Standards. The Westminster Confession of Faith has this very question at the very beginning:
Chapter I Of the Holy Scripture
I. Although the light of nature, and the works of creation and providence do so far manifest the goodness, wisdom, and power of God, as to leave men unexcusable; yet are they not sufficient to give that knowledge of God, and of His will, which is necessary unto salvation. Therefore it pleased the Lord, at sundry times, and in divers manners, to reveal Himself, and to declare that His will unto His Church; and afterwards for the better preserving and propagating of the truth, and for the more sure establishment and comfort of the Church against the corruption of the flesh, and the malice of Satan and of the world, to commit the same wholly unto writing; which makes the Holy Scripture to be most necessary; those former ways of God's revealing His will unto His people being now ceased.
 ROM 2:14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: 15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another; 1:19 Because that which may be known of God is manifest in them; for God hath shewed it unto them. 20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse. PSA 19:1 The heavens declare the glory of God; and the firmament sheweth his handiwork. 2 Day unto day uttereth speech, and night unto night sheweth knowledge. 3 There is no speech nor language, where their voice is not heard. ROM 1:32 Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them. 2:1 Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.
 1CO 1:21 For after that in the wisdom of God the world by wisdom knew not God, it pleased God by the foolishness of preaching to save them that believe. 2:13 Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual. 14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
 HEB 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets.
 PRO 22:19 That thy trust may be in the Lord, I have made known to thee this day, even to thee. 20 Have not I written to thee excellent things in counsels and knowledge, 21 That I might make thee know the certainty of the words of truth; that thou mightest answer the words of truth to them that send unto thee? LUK 1:3 It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus, 4 That thou mightest know the certainty of those things, wherein thou hast been instructed. ROM 15:4 For whatsoever things were written aforetime were written for our learning, that we through patience and comfort of the scriptures might have hope. MAT 4:4 But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. 7 Jesus said unto him, It is written again, Thou shalt not tempt the Lord thy God. 10 Then saith Jesus unto him, Get thee hence, Satan: for it is written, Thou shalt worship the Lord thy God, and him only shalt thou serve. ISA 8:19 And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? for the living to the dead? 20 To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.
 2TI 3:15 And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus. 2PE 1:19 We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts.
 HEB 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, 2 Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds.