Here are some sales that might be of interest.
Amazon has a 30% off sale of one item until 11/30. Use the code "HOLIDAY30." But do note that it caps out at $10 so you won't get a great break on more expensive volumes. If you don't have Prime then free shipping starts at $35. You can check it out here.
Barnes and Noble has a true 30% off sale on one item until 11/29. Use the code "30BFRIDAY." Free shipping starts at $25. You can check it out here.
Oxford University Press has 50% off of volumes here until 1/15/2016. On the downside, free shipping starts at $150. Standard shipping charges for US orders is $5.50 for the first book and $1.50 for each additional book You can check it out here.