Our Ecumenical Patriarchate in no uncertain terms prohibits the use of any and all electronic devices on the analogion (Patriarchal Encyclical Protocol No. 615), so physical books are necessary inside the church. But, printed books in the United States are hard to come by and expensive. Reprints are a great way to get around this issue, because (1) they are old and trusted books, approved by the Ecumenical Patriarchate, (2) can be ordered online through amazon, and (3) are cheap (no profit is being made on any one of the books below, the cost covers only the printing cost).
All of these books are public domain, no one really holds the copyrights to them.