Am I allowed to interrupt in the middle of Shmona esrei in order to pick up a Siddur (or any other holy book) that has fallen to the floor? Same question, if I have to make some steps in order to pick up a Siddur?
The Mishna Berurah (OC 96 sk 7) rules that if leaving the book on the floor will disturb your kavana (concentration), then you may pick it up in between the different blessings of shmoneh esrei. The Mishna Berura is not explicit about walking, though it does sound like he would permit it.
The Be'er Moshe 3:13:6 explicitly permits walking to pick it up if it is bugging you.