Going back to the original question...
Shattered Star always seems to get a bad rap. It's one of my favorite APs, and can get really nasty in places. It also goes to level 18.
I would also recommend that you take a look at 3PP Pathfinder modules. If you want a very tough adventure, Rappan Athuk and Slumbering Tsar are both deadly APs written (or converted) for Pathfinder.
Likewise, if you're looking for a creepy, dark fairy tale series of adventures, Tales of Old Margraeve from Kobold Press is very, very good.
If you want to run any 3PP adventure in PFRPG, you'll need to do a little bit of world conversion, but that's really not hard. The encounters are all ready to go.
As for supplemental materials...
For Reign of Winter, Irrisen: Land of Eternal Winter is pretty much a must-have. The first two books take place there, and I found it very helpful. You might want to pick up Cities of Golarion, which has a full gazetteer for Whitethrone, the capital of Irrisen. Book 2, The Dancing Hut takes place there, and there are many plot hooks in the module that reference stuff in the gazetteer. Distant Worlds is another Campaign Setting that is somewhat useful, as Book 4 takes place on the planet Triaxus.
Running the Serpent's Skull AP tallied up more kills on our group of players than any other AP I've run to date. I like how it spams saves vs. paralysis/poisons and supplements +2-3 CR encounters with TPK machine monsters *cough* shadow demon *cough*. That combined with the low WBL makes for a rough campaign.