From Chapter 1, some possible questions would be:

1. Why was Buck such a valuable dog at this time and place (he was big and strong, gold was discovered in Alaska and people needed that kind of dog...)
2. What was so important about the man in the red sweater?
3. Why was the man's club a "revelation" to Buck?

From Chapter 2, you could ask

1. What are two reasons why Curly's death was important for Buck's education and his future?
2. What makes Francois think Buck is so smart?
3. Why did Buck stealing bacon show that he was "fit to survive?"