It was written from George Wiley who was living in Pomona, CA to his friend William Hill in Casey, IL. George's daughter Nancy Jane Wiley had been married to William's son Charles.
These two old friends were in their 70's at the time (both having been born in 1838). They had both served in the Illinois Infantry - George in the 73rd, William in the 59th - during the Civil War, and had both been present at the battle of Missionary Ridge in 1863.
It is easy to imagine two older gentlemen talking about their health, their families, and reminiscing about their "glory days" when you read this letter. George talks about his health, asks about William's health. He reminisces about a visit from his friends to California and talks about the crops. He also mentions how "lonesom" he is after losing his wife Susan (in January 1916). William can probably sympathize since he was widowed 2 years earlier.
For more details about George Wiley and his friend William Hill, see their individual posts.