As an aside, there is no numeric order for the Cults books. Their release schedule follows what we think will work best for the success of the line, and it does not represent order of importance or what not.
Now if I were to number the books, it would probably be something like:
Mythology
Darkness
Water
Earth
Solar
Lightbringers
Lords of Terror
Horned God
Invisible God
Lunar Way
Prosopaedia
Probably a good thing they are not released in that order!
There no book for mysticism? Mysticism is not a magic system.
Which books will we be seeing East Isles, Teshnan, and Kralorelan deities in? The Solar Book has quite a few Teshnite and Kralorelan deities.