If I opened someone else’s Sefer, and a little while later the owner came in and found the spine partially broken, must I assume I caused the damage? Thanks
No, you do not need to assume it was you that broke the Sefer. At best you are in doubt (safek) and that relieves you from any financial responsibility. Please note, The Rav is assuming that the owner of the sefer is not demanding payment for damages rather you are questioning if you are required to compensate.