I've never personally read the book but going by what you have put, I would say the following: To be 'on someone's ass' means to be 'hounding' them. I realize in my explanation I have just used another idiom. Let me give an example: Say you've got a deadline approaching in your workplace for which you have something to submit.. The boss may be visiting you every 2 minutes asking "Are you nearly done?". This would be an example of 'hounding', or 'being on someone's ass'. As for the meaning of 'cracked', in this context it could be exchanged with the word 'joked'. To make a crack about something, essentially means to make fun of it (make a joke). I hope this helps!

I've never personally read the book but going by what you have put, I would say the following: To be 'on someone's ass' means to be 'hounding' them. I realize in my explanation I have just used another idiom. Let me give an example: Say you've got a deadline approaching in your workplace for which you have something to submit.. The boss may be visiting you every 2 minutes asking "Are you nearly done?". This would be an example of 'hounding', or 'being on someone's ass'. As for the meaning of 'cracked', in this context it could be exchanged with the word 'joked'. To make a crack about something, essentially means to make fun of it (make a joke). I hope this helps! (I realize I posted this in the comments section, I will move it to the answer section!)