It is neither a sin to seek knowledge about the books left out of the Bible, nor is it a sin to seek knowledge from them. Anyone who tries to dictate what you read, especially if they are using God and sin as a pretext, is someone you should get far away from.
What is Sin?
1 Jn 3:4 Whosoever commits sin, transgresses also the law: for sin is the transgression of the law.
Ro 5:13 For until the law, sin was in the world: but sin is not imputed when there is no law.
So, Sin is the transgression (in the sense of missing the mark) of the Law and where there is no Law there is no sin. Nothing in the Law dictates what you can and cannot read.
Having said that, there are many books left out of the Bible containing valid scripture. These books are mostly available at your local bookstore. Some examples are The Apocrypha, The Dead Sea Scrolls, The Nag Hammadi Library, The Book of Enoch and The Book of Jasher… for starters. Excluding these books from a study of God’s Word is like reading the cliffs notes instead of the actual book. Some of these books clarify teachings of the Bible while others are clearly not in line with God. The only way to know is to study them.
2 Ti 2:15 Study to show yourself approved unto God, a workman that needs not to be ashamed, rightly dividing the Word of Truth.
Pr 15:28 The heart of the righteous studies to answer: but the mouth of the wicked pours out evil things.
There is a wrong way of dividing the Truth. Unless you study to find the answer, you might be fooled.
The Book of Jasher as a great place to start. Jasher follows along with Genesis and Exodus up to the taking of the Promised Land. It is also mentioned, in the Bible, in Joshua 10:13, 2 Samuel 1:18, and 2 Timothy 3:8.