User blog comment:Guezz WUT24/Rumble's Revenge Cryptograms/@comment-70.133.145.6-20130121183555/@comment-2232235-20130123001907

Maybe the secret society is supposed to keep people from finding out the mysteries of Gravity Falls. Which is why Stan, being a member, denies all the weirdness whatsoever toward Dipper and Mabel.

So Stan and/or Bill could be after the books, since they list most of the town's enigmas and obscurities.