Allowed Splatbooks
Hey there, Kellons! :3
Akellon's stance on splatbooks is pretty loose. Pretty much anything is open, though always check with your DM if you want something really esoteric or strange, or if you're just not sure.
The one major exception to this is PSIONICS. At this point in Akellon's timeline, psionics do not exist on the continent, and will not exist until at least the late 1300's CA. No psionic books are currently open, as the current campaigns take place in the early 1200's CA.
Other than that, have fun!