By Aharon Gabbay

Maran Harav Yosef Karo writes in Orach Chaim 14/4 that one may take his fellows tallit (to use for prayers) without permission providing he folds it again, if he found it folded. The Mishnah Berurah explains that the reason this is allowed is due to the fact that your average Jew is content that you would be using his item for a mitsvah (even if you didn’t ask him). However the Mishnah berurah adds that it is only permitted if the borrowing is a one off. Furthermore, the Mishnah Berurah also says that the case we are referring to where its usually found (i.e. in shul), but it would not be permitted to enter someone’s house and take their tallit, that would be considered stealing. The Mishnah berurah says that if its possible to ask the owner then it should be done, especially if the owner is particular with his items.

Rav Moshe Feinstein adds that despite the fact that a woman is not obligated to wear a tallit (so one might say that the owner wouldn’t necessarily want her to take it), still this previous ruling would apply and she could use the tallit (if she pleases). Rav Moshe also says that it would also be allowed to take a child’s tallit (even though he might not understand what mitsvot are).

Furthermore, the source for this entire ruling, the The Beit Chadash, also says that it similarly it would be allowed to borrow teffilin or arbah minim from someone else without permission, but again, it should only be done as a one off.

Rav Moshe Isserliss says that when it comes to borrowing a fellows siddur or sefer there is no permission because, explains the Mishnah Beruruah, that you might use it for too long and come to rip a page and so it is not allowed, even as a one off. It would seem (to me) that the same applies with your average harry potter book.

Shabbat Shalom