Is it ok to place the sefer that you're learning from slanted on top of another sefer/sforim - sort of like a shtender or is is this considered to be a zilzul (disrespectful) to the sforim?
This is a shailo that is disputed among the achronim. The Taz in Yoreh Deah (as pointed out by @DoubleAA) says that it's ossur. However see OC Siman 154 in the M"A Sif-Katan 14 that seems to say that this would be muter. See also the M"B in Siman 315 Sif-Katan 30 that brings the Taz however he ends off with the M"A that is makil along with the Chaiya Adam in Clal 31 Sif 48. The Kitzur Shulchan Aruch is also matir (what I have written down in my M"B, however the exact source I will have to look for later.)
The Taz to YD 282:19 writes:
נראה לי דאותן אנשים שנוהגים בשעת לימודם בספר ורוצה להגביה הספר שלומד פושט ידו ולוקח ספר אחר ומניח תחת זה שלומד דאיסורא איכא משום בזיון...כי בזה מבזה הספר להיות לתשמיש מה שיוכל לעשות בעץ או באבן...והוי בזיון גדול כל שהוא מביאו ממקום למקום אפילו בשלחן אחד אם לא שהספר התחתון מונח כבר ודאי שרי להניח השני עליו:
It seems to me that those people who are accustomed while studying a book and wanting to lift the book higher to reach out and take another book and put it underneath the one they are studying from, this is forbidden because of disgrace [to the book]...for by doing so he disgraces the book to use it as merely wood or stone...and it is a big disgrace if they bring the book over for this purpose. However, if the book was already sitting there, then they can place another book on top and not worry about it.