Is a Sefer as a Shtender ok?
There is a Machloikes between the Mogen Avrohom and the Taz whether one is allowed to put a Sefer under another Sefer in order to lift the latter up so that it should be more comfortable to learn from.
Since this is a disagreement related to Kovod HaSeforim, respect to Seforim, of course one should be strict and not do what is commonly done, piling up a few Seforim on top of each other in order to elevate the top?most Sefer or Siddur to a more comfortable height.
If the Sefer is already there, that is a different Halacha:
Once a Sefer is already there and one puts another Sefer or Siddur on top of it, there is absolutely no problem and that is permissible.
Otherwise, taking a Sefer from elsewhere and using it as a Shtender for another Sefer or Siddur is unacceptable.