I was wondering is it a sin to possess books of knowledge of other religions and practices such as a spell book or other religious materials just to satisfy my curiosity?
I find nothing that says it is a sin to possess books. I hope it isn’t, since I own copies of the Qu’ran, many books on Judaism, the Communist Manifesto, the Book of Mormon, and others. I would dearly love to own a copy of the so-called Book of the Dead of ancient Egypt. (In addition to what I responded to this questioner I would point out that there is value in knowing the beliefs of other religions which you can only truly get from their books. It is easier to convert somebody if you know the similarities and differences between your beliefs.)
The closest thing I can find in the Bible to this subject is in Acts 19:19. “Many of them also which used curious arts brought their books together, and burned them before all men: and they conted the price of them, and found it fifty thousand pieces of silver.” This was an example, though, of people who had used those books showing their rejection of their former ways. It does not indicate that we can not own such books.