Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
45. It is a positive commandment to marry the wife of one's brother if he dies without children
As it states in Devarim (Deuteronomy) 25:5, "If brothers live together and one of them dies, the widowed wife's brother-in-law (her dead husband's brother) shall take her to be his wife." In our times, since we do not have the right mindset to fulfill this commandment only for the sake of establishing the name of the dead brother, this law is no longer in force. Instead, Chalitzah is performed, as explained in the next mitzvah.