Yalkut Meam Loez (Bereishis vol.1 p.123) urges us to daven while looking in the siddur. He writes, "When you daven by heart, your mind will wander and think about other matters. But if you daven while looking in a siddur and don't raise your eyes, you are guaranteed not to be distracted by foreign thoughts. This is because when you toil with all your strength to daven with kavanah, Hakadosh Baruch Hu will certainly have compassion on you and save you from the yetzer hara."
In particular, the sefarim discuss saying birkas hamazon from a bencher. The Yalkut Meam Loez writes, "The Arizal would even say birkas hamazon from a sefer, although birkas hamazon is something that even children know by heart. And the reason is what we wrote above."
Reb Eliezer ben Makir, in his sefer Seder HaYom, when discussing Birkas HaMazon, writes, "If you have a sefer before you to look in, you can have better kavanah, and you won't think about other matters. That is certainly very good. You will be keeping one of the mitzvos of the Torah properly, bringing blessings to yourself and your entire family."
A woman came to the house of Reb Yehoshua Leib Diskin zt'l and told him her immense tzaar that one of her children died young, r'l. She requested a brachah to bear another child. Reb Yehoshua Leib advised her to accept on herself to say birkas hamazon from a siddur. The woman did so, and she had another child. (Amud Aish Toldos HaGaon Maharil Diskin p.161)
