Leaving a Sefer open:
One is not to have a Torah book [i.e. Sefer] remain open if he leaves the room, unless the Sefer is covered by a cloth and the like. According to Kaballah, one who leaves a Sefer open and exits the room causes a certain angel by the name of “ש”ד”, which stands for “Shomer Dapin/guards pages”, to make one forget his learning.
What items may be placed on top of the open Sefer?
May one place another Sefer on top of the open Sefer in order to cover it?
Some write it is permitted to cover an open Sefer with another Sefer. This may be done even if the lower Sefer is a Chumash and the upper Sefer which is being placed on it is of Torah Shebal Peh. However, other Poskim are stringent to prohibit covering an open Sefer with another Sefer, and the only option available is to use a cloth or close it. Practically, the custom is like the former approach.
How much of the Sefer must be covered?
The custom is to cover some part of the Sefer, and not require the entire Sefer to be covered.
May one leave the Sefer open and leave the room if someone is guarding the Sefer?
Must the Sefer be covered if one will be returning shortly?
Some Poskim rule that if one will only be leaving the room for a short amount of time, such as to go to the bathroom and the like, there is no need to close or cover the Sefer. Other Poskim however rule one must cover the Sefer even in such a case.
Must the Sefer be covered if one will be in the room next door?
Some Poskim rule that one is only required to close or cover the Sefer if he will be leaving the house, however if he will be going to a room next door then there is no need to do so.
May one leave a Sefer open for the sake of the Sefer, such as for it to dry if water spilled on it?
 Michaber Y.D. 277:1 regarding Sefer Torah; Bach Y.D. 277:1 based on Yerushalmi, brought in Shach 277:1 that the same applies for all Sefarim; Taz 277:1 in name of Bach ibid; Shach ibid based on Eiruvin 98a; Shiyurei Kneses Hagedola 282; Mishnas Chachamim Reish; Reishis Chochma Shaar Hayirah 15; Birkeiy Yosef 277:2; Kitzur SHU”A 27:4; Yeshuos Chochma 27:49-50; Yados Nedarim 277; Aruch Hashulchan 277:2; See Mikdash Me’at 277; Avnei Yashpa 1:202; Ginzei Hakodesh 2:27
 The reason: This is due to Bizayon, belittlement, of the Sefer. [Aruch Hashulchan ibid]
 Shach ibid; Kitzur SHU”A 27:4; Yeshuos Chochma 27:49-50; Aruch Hashulchan ibid
 Shiyurei Kneses Hagedola 282 in name of Sefer Chassidim
 Mishnas Chachaimim Os Reish that the Kneses Hagedola would cover it using a silk garment; Reishis Chochma ibid “Cover it with an important garment”
 Ginzei Hakodesh ibid, based on Sefer Chassidim 909 who writes one may place a Sefer of Torah Shebaal Peh on top of a Chumash to protect it from getting dusty.
 The reason: This is not similar to the disputed law regarding if one may move a Sefer for the sake of
 See Sefer Chassidim ibid
 Yeshuos Chochma 27:49-50 based on Pischeiy Teshuvah 282 in name of Chamudei Daniel that one may not use a Sefer as a bookmark for another Sefer; See Aruch Hashulchan ibid who does not give the option of covering it with a Sefer
 See Salmas Chaim 377
 Salmas Chaim 376
 Aruch Hashulchan ibid; Yados Nedarim 277 that so was his custom to leave the Sefer open; Avnei Yashpe 1:202
 Kaf Hachaim [Falagi] 20:11; Yaskil Avdi Y.D. 5:56
 Aruch Hashulchan ibid
 Pischeiy Teshuvah 277 in name of Chamudei Daniel; Aruch Hashulchan 277:3