That coupon caused me to spend a lot more than I had planned this month, which, I guess, is the very purpose of it.
I bought ten books from the "spirituality" category and nine on various writing topics. Unlike previous coupons, the 50% one finally worked with Zoe Fairbairns' "Write Short Stories" (second time I'm buying this: the iBooks version couldn't be liberated, and it's a book I really treasure). Oh, and I picked up the handbook for LMMS (music studio software) that was a tad too costly without the code.
I resisted buying any fiction with the coupon, though, since I do have a huge backlog, so I limit myself to purchasing only text/reference books for the time being. Not that it makes an actual difference, since novels and anthologies don't exactly go "bad" either, but I find the reference-style books easier to justify. "Education" makes a better excuse than "entertainment".
I also pre-ordered a programming book at the Penny Press, which also had a 50% discount.
There are quite a few paper books on my shelves that I would like to replace with e-book versions, but it's something I struggle with. It's not strictly necessary, isn't as powerfully satisfying as newer-read-before books whose purchase makes you glee like a kid in a candy store, and it just feels a little like wasting money.