From Ray's site:
Due to popular demand, and a few new titles, I'll be doing a September 9, 2007 Galore Park / Horrible Books Order. So, I will be accepting Galore Park / Horrible Books Orders anytime now until midnight Sunday, September 9, 2007. I will be at a conference and camping in the redwoods through August 25, 2007, with limited email capabilities, but will get back to you as best I can, and for sure on August 26, 2007. :)
You can read about the different Horrible Histories titles at the UK Amazon site. Murderous Maths are here. The UK Scholastic site has more about the different book series offered by Ray, as well. Ray gets good prices on these books, which can be hard to find in the US, and his shipping is a flat rate of $7 no matter how many books you order.
And don't sweat it. I plan to do this again on the following dates : October 14, 2007, January 27, 2008, May 25, 2008, and October 19, 2008 for your Galorible Reading Pleasure!