Using someone’s Sefarim without permission:
It is forbidden for one to learn from Sefarim that belong to one’s friend without his permission. This applies even if one only plans to learn from it on a mere occasion. This applies even if one is certain that no damage will occur to the Sefer while reading it. It is thus forbidden for one to visit his friend’s house in order to read a Sefer that is there without his permission even on mere occasion. This applies even if the Sefer belongs to a third person and was borrowed from him.
One knows for certain that the owner is not particular: If one knows for certain that the owner is not particular against one using his Sefarim, then it is permitted to use it without his explicit permission. Thus, one who gives Sefarim to a Torah scholar for him to hold and guard on his behalf [i.e. Pikadon], it is permitted for him to read from the Sefer even without the owner’s permission, as certainly the owner gave it to him having in mind that he will learn from it.
Sefarim that people are not generally particular against being used: Those Sefarim which it is not common at all for any people to be particular against someone using without their permission, it is permitted to use these Sefarim without permission. However, those Sefarim which minorities of peoples are particular against them being used by others due to worry of damage, then it is forbidden to be used without permission even if majority of people are not particular. This applies even if one is positive that no damage will occur to the Sefer at all, it is nevertheless forbidden to borrow without permission. [Practically, today, based on this a number of leniencies have been adapted regrading using other people’s Sefarim without permission as explained in the Q&A!]
It is forbidden to use someone else’s Sefer without permission, unless one is certain the person is not particular against one doing so, or it is not common at all for any person to be particular.
May one in Shul use someone else’s Siddur without permission?
Many are accustomed to use another’s Siddur without permission. Some Poskim question this allowance, as Siddurim should maintain the same law as Sefarim. Other Poskim however defend this practice, stating that today people are not at all particular against this, especially in light of how cheap and easily attainable these Sefarim are today. It is however forbidden to remove the Siddur from the Shul without permission from the owner.
May one use someone else’s Sefer left in a Shul without his permission?
Some Poskim rule it is forbidden to use Sefarim that are found in Shul without the permission of the owner, just as is the law regarding Sefarim that are in his home. Nevertheless, since people are accustomed to do so, the owners are to declare and publicize that they allow others to use their Sefer that is left there. Other Poskim however rule it is permitted to use someone’s Sefer that was left in the Beis Midrash without his permission, as Sefarim are inexpensive and one can assume the owner left it there for others to use. Practically, the widespread custom today is to allow looking into someone else’s Seafrim without his permission, although it is best for the owners to write explicit permission of use on the Sefer. In all cases, it is forbidden to remove the Sefer from the Shul without permission from the owner.
May one take a mere glance into someone else’s Sefer to look something up, without his permission?
The widespread custom today is to allow one to have a mere glance to look something up in the Sefer.
 Admur O.C. 14:10 and C.M. Metzia Upikadon 27 and Sheila Vesichirus 5; Rama O.C. 14:4; Smeh C.M. 72:6; Nimukei Yosef Tzitzis 12a in name of Ritva; See Piskeiy Teshuvos 14:10
 The reason: As the owner of the Sefarim suspects that perhaps he will spend a long time reading from it to the point that they will tear due to over usage, and therefore he is not pleased with allowing another to read from it at all. [Admur 14:10 and C.M. Metzia Upikadon 27; Nimukei Yosef Tzitzis 12a in name of Ritva]
 Admur ibid; M”A 14:10; Chesed Lealafim 14:5; M”B 14:16
 Admur C.M. Metzia Upikadon 27 and Sheila Vesichirus 5 regarding all objects
 Admur Sheila Vesichirus 5;
 Admur Sheila Vesichirus 5; Rama C.M. 292:20 regarding Pikadon of Sefarim by Talmid Chachaim; Smeh 292:45
 Rama C.M. 292:20; Smeh 292:45; Admur 14:10 references to this Halacha
 Admur Sheila Vesichirus 5; Shita Mekubetzes Baba Metzia 22b
 The reason: Although majority of people are not particular due to that chance of damage is very rare, nevertheless, we do not follow the majority in this regard to say that one can assume that this owner is not particular against it being used. [Admur ibid]
 Admur ibid and C.M. Metzia Upikadon 27 in parentheses regarding Sefarim
The reason: As nevertheless, the owner of the item suspects that perhaps it may get damaged, and thus if he knew that this person was using his item, it is possible that he would be particular, and it is thus found that he has borrowed without permission of the owners [Sheol Shelo Midaas], which is considered stealing, even if one causes no damage or loss to the item at all, as explained in Hilchos Metzia Upikadon 27. [Admur ibid; Metzia Upikadon ibid in parentheses]
 P”M 14 M”Z 7; M”B 14:16;
 Makor Chaim 14; Likkutei Maharich Birchas Hashachar; Aruch Hashulchan 14:13; Piskeiy Teshuvos 14:10
 Rama ibid; Admur 14:10 and 12 regrading Tallis and Tefillin
 P”M 14 M”Z 7; Chesed Lealafim 14:5, brought in Kaf Hachaim 14:31, that it is in truth forbidden to do so and those who do so are receiving an Aveira instead of a Mitzvah.
 Chesed Lealafim ibid
 See Rama C.M. 163 in name of Takanos Kadmonim and Biur Hgar”a there; Minchas Yitzchak 7:130; Piskeiy Teshuvos 14:10; See Chesed Lealafim ibid, brought in Kaf Hachaim ibid, for a similar defense regarding Siddurim; Lehoros Nasan 6:127-128
 Aruch Hashulchan 14:13
 Rama ibid; Admur 14:10 and 12 regarding Tallis and Tefillin
 Aruch Hashulchan 14:13; See Sdei Chemed Samech 15 who questions this ruling