depends on your casebook.
The "Understanding" book is pretty good for discussing the Unocal/poison pill cases. If you're not already knowledgable about the corporate structure, Unocal et al will be the most difficult part of any of your upper level classes.
I thought Gilbert's would have been helpful, but it wasn't geared towards the casebook I was assigned. The Bauman casebook sucked. If you're using it, I suggest changing professors.