(5) Tree Line
You might be tempted to read this novel as a story about two generations of fathers and sons. Be forewarned, though: this book has a lot more going on than some bonding issues. It wouldn't hurt to refresh on your first few books of Genesis—or wait until Chapter 22, when Lee just reads it to you.
The thing that makes this book kind of strange is that, beyond just reliving the Cain and Abel story, the characters in the novel are aware that they are reliving the Cain and Abel story. Keep your ears perked for really in-your-face allegorical moments (like how Charles and Cal—the Cain stand-ins—like to farm, or how Adam talks about turning his ranch into Eden with Cathy/Eve), and think about how these meta-moments help drive the story.
Language-wise, this novel is a walk in the park. Narrative-wise, it is easy to follow, though it does jump around between the Trask plotline and the Hamilton plotline. Pro tip: Keep track of how much time has passed between chapters, and look for historical clues like the campaign against the Native Americans, the turn of the century, and World War I.
Oh, and about halfway through the book Cathy becomes Kate. Just deal with it.