Q. Why were some books removed from the Bible and is it a sin to read them?
I believe you are talking about the so-called Apocrypha. That term refers to books that were written in Greek within the Jewish community in the centuries before Christ. Those books are distinct from the Old Testament because they were written in Greek, not Hebrew, and they are distinct from the New Testament because they were written before Christ came, not after. So there is already something about them that sets them apart as different from the books that all Christians accept as inspired Scripture.
Nevertheless, after lengthy discussion and debate in the few centuries after Christ, regional councils in the western part of the Roman Empire, at Hipppo in 393 and Carthage in 397, approved adding these books to the canon of Scripture, as long as this decision was ratified by the central authority in Rome.
No action was taken in that regard for over 1,000 years. But finally, in 1546, the Council of Trent, largely in response to the way Martin Luther had separated out these apocryphal books and placed them between the testaments in his German translation of Bible, decreed that they were as fully canonical as the others. Nevertheless, the Roman Catholic church still describes these books as deuterocanonical, meaning that they belong to a second group of books “whose Scriptural character was contested in some quarters,” as opposed to the protocanonical books, the collection of “sacred writings which have been always received by Christendom without dispute.”
The Council of Trent also decreed that the Vulgate was the authoritative text of Scripture. That actually sent something of a mixed message about the Apocrypha, because St. Jerome’s prologues were always included in the Vulgate, and in his prologue to the book of Kings, in which he surveyed the entire Old Testament, he specified that the books that had been translated from Greek, rather than from Hebrew, are “set aside among the apocrypha” (inter apocrifa seponendum) and “are not in the canon” (non sunt in canone). He made similar comments in the prologues to several of the apocryphal books themselves. So while the Roman Catholic Church’s embrace of these books is explicit, its position on them is not without internal tensions.
Eastern Orthodox Bibles include all the books in the Catholic Apocrypha along with several more. However, it classifies all these apocryphal books as Anagignoskomena (“worthy to be read”), meaning that they are read during services of worship, but that they are not as authoritative as the other books. Orthodox theologians sometimes call the apocryphal books deuterocanonical to indicate their secondary authority, using this term differently from Catholics, for whom it describes how these books were received after first being disputed.
And Protestants, ever since Martin Luther, have not considered the Apocrypha canonical, except for Protestants in the Anglican/Episcopalian tradition.
So maybe the real question is not why some books were removed from the Bible, but why some books that were different from both the Old Testament and the New Testament were added to the Bible. The answer is that, as the Eastern Orthodox say, they are “worthy to be read.” They provide important information about what happened in the years between the testaments, they tell inspiring stories of how people remained faithful to God during difficult trials in those times, and they add to the collection of wise advice for living that is found in the canonical wisdom books.
So it is certainly not a sin to read them. Even Protestants, who do not consider them to be inspired Scripture, say that they are edifying, meaning that reading them can strengthen our faith and devotion to God. As a Protestant myself, I do not have these apocryphal books in the Bibles that I use regularly for study and devotions. But I do have copies of these books in some other Bibles that I own. I have read the apocryphal books and gotten a lot out of them.
I hope this provides you with some helpful background to the issue. As I said, it would certainly not be a sin to read those books, and I think they would help you learn some useful things if you did read them. If you belong to a community of Christians, and if this issue is important within that community, you could explain to anyone you told about reading the books that you were not reading them as Scripture, but as edifying literature that has come down to us from within the tradition of our faith. I hope no one would be upset about that.