Kitzur Shulchan Aruch (28:9) writes:
לא ישרטט איזה קונטרס על הספר, לפי שאין קדושה בקונטרס, עד שיכתבו בו
You should not draw lines on notebook [while] on [a holy] book, because the notebook is not sacred until [something sacred] is written on it
Seemingly, once one has written in the notebook, it would be permitted to place it on top of other sefarim. Are there any sources that says so (or against doing so) explicitly?