Gemara

(Mishnah): If one was reading a Sefer [of rolled up parchment] on the roof, and it unraveled and reached within 10 Tefachim of the ground, he turns it Al Panav (so the writing faces down);

R. Yehudah says, if the end is any amount above the ground, he may roll it back;

R. Shimon says, even if the end is on the ground, he may roll it back, for disgrace to Kisvei ha'Kodesh overrides mid'Rabanan laws.

98a - Question: One may not turn Kisvei ha'Kodesh Al Panav!

(Beraisa): One who writes a Sefer, Tefilin or Mezuzah may not turn it Al Panav. Rather, he spreads a garment over it.

Answer: There, there is an alternative. Here, if he will not turn it Al Panav, there will be even more disgrace to Kisvei ha'Kodesh!

(Rav Acha bar Ahavah): We do not throw Kisvei ha'Kodesh.

Shabbos 115a (Mishnah): One may save all Kisvei ha'Kodesh from a fire [on Shabbos], whether or not we read in them;

Even if they are written in another language, they require Genizah (burial).

115b (Beraisa): One may not save Berachos from a fire. Therefore, one who writes Berachos is like one who burns Torah.

Once, R. Yishmael heard about a man who wrote Berachos. The man sensed that R. Yishmael was coming to inspect, and stashed loads of them in a flask of water.

R. Yishmael: You will be punished more for this than for writing them!

Rishonim

Rambam (Hilchos Tefilin and Sefer Torah 10:5): One may not throw any Kisvei ha'Kodesh, even Halachos, and Agados.

Rambam (Hilchos Yesodei ha'Torah 8:6): One may not burn or destroy any Kisvei ha'Kodesh or explanations of them. One who does so is lashed mid'Rabanan. All Kisvei ha'Kodesh that wore out are buried.

Rashi (98a DH Lo): He turns the Sefer Al Panav because there is no alternative, i.e. he has no garment to spread on it.

Poskim

Shulchan Aruch (YD 277:1): If one wrote a parchment [for a Sefer Torah] and wants to leave it to dry, he may not turn it Al Panav, even though he intends to prevent dust from gathering on the writing. In any case, this is disgraceful. Rather, the writing must face up, and he spreads a garment over it.

Bach (1): Rashi clearly says that when it is impossible, e.g. he has no garment or similar matter, it is better that the writing face down. If not, there is more disgrace to Kisvei ha'Kodesh. The Yerushalmi says that if one has no cloth for a Sefer, he inverts it. This shows that one should not go outside and leave a Sefer open if he has no cloth. This is not only for a Sefer Torah. The same applies to other Seforim.

Shach (1): The Bach did not need to learn from the Yerushalmi. This is clear from the Bavli in Eruvin! Chachmei Kabalah know that there is angel called Shed (SHomer Dapin). If one leaves a Sefer open and leaves, [the Shed causes that] he forgets his learning.

Nekudas ha'Kesef: I learned from the Taz that the Bach actually learns from the Bavli. "This shows that" refers back to the first matter he said.

Aruch ha'Shulchan (2): The custom is to be careful to close the Sefer or cover it. However, if one goes only from room to room, or even to the street for a short time, there is no concern.

She'elas Rav (2:14:17): Ha'Gaon R. C. Kanievsky Shlita is lenient to leave a Sefer open if he does not leave the room.

Ginzei ha'Kodesh (p.260(28), 272(5), citing ha'Gaon R. C. Kanievsky Shlita): The custom is to cover it when leaving the room. It is better to cover it with a hat or Gartel than to close it [for a short time].

Ginzei ha'Kodesh (2(50), citing ha'Gaon R. Shmuel Auerbach, Shlita): All permit leaving it open if he cannot mark his place, and if he closes it he will lose time from learning to find his place.

Birkei Yosef (2): One should beautify Seforim, and place them and store them in a choice place, and cover them with a distinguished Talis and guard them from mice. He moves them honorably and modestly, like one who moves the king's garments in front of the king. He touches them only with clean hands (Ri ben Shushan, brought in Reishis Chachmah 15). This shows that we should be Mevatel the custom to leave a Sefer open on the Kisei Eliyahu a day or two before a Bris Milah. This disgraces the Torah.

Pischei Teshuvah (1): Chamudei Daniel says that if a Sefer Torah began to wear out, and he wants to spread it to air out to preserve it, he turns it Al Panav. If he cannot, it is permitted even if the writing faces up. Whatever is for its need is permitted.

Shulchan Aruch (282:5): One may not throw Kisvei ha'Kodesh, even Halachos, and Agados. One may not turn them Al Panav. If one finds them like this, he must invert them.

Gra (15): The Mishnah discusses one who was reading a Sefer. This includes all Seforim.

Shiyurei Berachah (of Birkei Yosef, 5, citing Radak): Many are not careful about this. When they are called to the Sefer Torah, or even when they cease learning for worldly matters, they turn over the Sefer from which they learn. They will be punished for this. It will be pleasant for those who rebuke them.

Avnei Yashfe (3:94:1, and Ginzei ha'Kodesh p.289): Ha'Gaon Rav Y.S. Elyashiv Ztz"l said not to turn over a Sefer because it seems disrespectful, but it is not so severe.

Gilyon Maharsha: Some bookbinders use pages from old Seforim (to make covers for Seforim). This is a whopping sin.

Be'er Sheva (43): Many people burn Kisvei ha'Kodesh that wore out or were erased. They say that this is their honor, for through burial, they are cast all over in filthy places. Rashi says that one may not save Berachos that Chachamim enacted, e.g. Shemoneh Esre, because they were not allowed to be written, for one may not write oral Torah. Tosfos, the Rosh, the Mordechai and Hagahos Maimoniyos say that nowadays that forgetting is rampant, we may write and read everything, even Kesuvim in every language, due to "Es La'asos la'Shem Heferu Torasecha" [lest Torah be forgotten]. Therefore, nowadays we save everything from a fire. This shows that one may not burn them or overtly destroy them, even those that one may not write or read. Even things that require Genizah, which one may not save from a fire, one may not leave it in a Hefker place. The Rambam forbids burning or destroying all Kisvei ha'Kodesh, and requires burying those that wore out.

Suggestion: If the Rambam includes oral Torah, and even Halachos and Agados, he should have specified, like he did at the end of Hilchos Sefer Torah!

Rebuttal (Be'er Sheva): Also here, he included the explanations of Kisvei ha'Kodesh. Also, it is clear from the Gemara that one may not destroy even what is forbidden to write. He specified Halachos and Agados regarding the Isur to throw, for there the Gemara discussed only what was permitted to write then, i.e. written Torah. The Rambam extrapolated that nowadays that we may write everything, we may not throw any Divrei Torah. If people understood the law of Genizah, they would not be lenient. We bury a Sefer Torah in an earthenware Kli, next to a Talmid Chacham (Megilah 26b). The Ran (8b DH uvi'Chli) says that we do as much as possible to prevent rapid decay. It should not look like overt destruction. All the more so one may not bury Seforim where they will be cast around in filthy places. This is like overt disgrace! The Gemara taught we do not throw Kisvei ha'Kodesh. We do not discuss Resha'im who would throw them on the ground! Rather, one should not throw them to someone. All the more so they should not be cast on the ground, even in a clean place. This is the ultimate disgrace.