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.…