Bookwalk Coin Up (Free Book Deal Dead Though).
Edit: Coin up still active, but you earn less than the book's cost now.
So, Bookwalker has a deal going on with Coin Up right now on Kadokawa stuff.
This is going on concurrently with a sale on select collected volume works. Every coin you spend here on these collected works, you'll get back (check the coin up to be sure though).
The only catch is, you aren't allowed to spend these coins immediately. I'm not sure if they don't grant them until next month or no. But if you've got some excess coins hanging around, or plan to buy some for other works, it's a good way to get something without actually losing them.
Edit: The coins are granted fairly quickly actually. So yeah, get those series guys. O.o
This appears to have been fixed if it was in error. At least the last few volumes I clicked on don't offer equal coin up to cost once added to the cart (it shows equal coins on the item page, so be careful).
I managed to snag a few including Omawari Himari and Sundays Without God. =]