Amazon is offering 25% off any one book today through December 14, 2015 by entering code 25OFFBOOK at checkout.
There are many ideas for great buys but here is one of the best:
The entire Harry Potter book series is currently on sale for $45.84. Get it at an additional 25% off, plus get free shipping! After entering the code, you get the whole set for only $39.33 (which includes tax for my state so prices may vary slightly).