Gen 15-17 covers God promising Abram a son, Sarai and Hagar and the Covenant of Circumcision. Some interesting notes regarding this section:
Abram was growing more and more upset with God because he did not yet have a son. Abram could not understand how God had promised him all these descendants, and yet he was growing old and still had no son; the theme of faith is one of the major themes of not only Abram's life, but the entire Bible.
The reason, in v. 10 that the animals were cut in two is because it was customary when two people made a covenant, they would walk between the two halves with the understanding that should one of the parties break the covenant, the same would happen to them.
Sarai offering up one of her servants to her husband to have sex with in order to conceive a child was not something unusual. Based on the text and other history books one can see that this was a custom of the day. It, however, still fell outside of God's plan, as he had promised Abram and Sarai a son of their own. Once again, God requires faith of Abram and Sarai but instead they try and do things their way, which resulted in very negative consequences.
Ishmael (Hagar and Abram's son) means "God hears". Later on, when God tells Abram that He will indeed fulfill his promise and give him and Sarai a son, Abram (now Abraham) laughs. God tells Abraham to name his son "Isaac" which means "he laughs" to remind Abraham of his reaction that day.
In Ch. 17 JM notes that the reason God chooses circumcision was two fold: Firstly, it had clear health benefits which was important since God was going to build his nation from this group of people (Jewish women have historically had a much lower rate of reproductive disease and cervical cancer). Secondly, the male organ was the symbol of human sin and depravity since the "seed of sin" was carried by man and circumcision was symbolic of the need to cut away sin.
God renames Abram Abraham which means "father of many nations" and Sarai (my princess) to Sarah (princess) since she would be the mother of many nations.
