Question: May one place one Sefer on top of or lean it against another Sefer so that it is easier to read from the Sefer?
Answer: Some have the custom to take one Sefer, such as a book of Mishnayot, and lean it on an angle against another Sefer, such as a Siddur, so that it easier to read from the Sefer one is using.
The Turei Zahav (Yoreh Deah, Chapter 282, Subsection 13) writes that it is forbidden to take one Sefer to prop up another Sefer so that it is easier to read from, for this is disrespectful. Only if the other Sefer was already placed there beforehand is it permissible to lay another Sefer on top of it.
This means that according to the Turei Zahav, one may not use a Sefer as a stand for another Sefer. Only if the Sefer was already there beforehand is this permitted. However, to take a Sefer specifically for this purpose is disrespectful to the Sefer Kodesh.
Nevertheless, the Magen Avraham (Orach Chaim, Chapter 154, Subsection 14) disagrees with the opinion of the Turei Zahav and permits doing so, especially if one is doing so in order to study Torah, in which case this is not disrespectful at all. He proceeds to prove his point from the words of the Sefer Chassidim who writes that if one is sitting and cannot learn because the sun’s rays are shining on the letters of one’s Sefer, one may take another Sefer and hold it up against the sun in order to provide some shade so that one may continue learning (as long as one intends to do so strictly for the purpose of Torah study and not for one’s personal comfort).
Maran Rabbeinu Ovadia Yosef zt”l writes in his Chazon Ovadia-Shabbat, Part 5 (page 321) that other great Acharonim concur with the opinion of the Magen Avraham and he proceeds to support their opinion.
Thus, halachically speaking, one may use one Sefer to prop up another Sefer in order to be able to study from it.