If one finds another’s sefer in a Beis Medrash, not as an avaida, but on a shelf where people keep private sefarim, i.e. in a yeshiva, is one allowed to be shoel shelo medaas in order to learn from it?
Although Shulchan Aruch and Mishnah Berurah write that it is prohibited to use another’s books, nowadays the general custom is that one may use another’s books unless he has explicitly forbidden their use by writing a warning in the front cover of the book or in another visible place. The reason for this change is that today’s books cannot be compared to those of yore — they are strong, they are fairly cheap, and they are easily replacable — all in contrast with books of times gone by. Therefore, unless stated otherwise, it is permitted to use a private book on the shelf. [See in a similar vein Minchas Asher, Parshas Shelach.]