I've been reading some apocryphal books, like gnostic books, and was enjoying them as they seem so unusual and otherworldly to me. I'm not a devoted Christian but, if I'd been a devoted Christian would be reading those apocryphal books a sin? If yes, how should I go about them?
The definition of sin is to miss the mark. So then the question is would reading those apocryphal books be missing the mark? I think that depends on your intent. If one reads those books and places their faith in them, then, yes, that is missing the mark. If one reads those books in order to learn why some people believe in them so that one can have meaningful dialog with those people in order to show them Christ, then, no, I don't think that is missing the mark. Acts 17:22 Then Paul stood in the midst of Mars' hill, and said, Ye men of Athens, I perceive that in all things ye are too superstitious. Act 17:23 For as I passed by, and beheld your devotions, I found an altar with this inscription, TO THE UNKNOWN GOD. Whom therefore ye ignorantly worship, him declare I unto you.
Paul spoke to the men of Athens and used one of their own alter's inscription to witness to them. That isn't a sin.