We've decided to release a free book of spells for Sertorius called Book of the Archon. Part of why it will be free is we don't want to charge extra money for something certain groups consider essential to play (having to buy tons of supplements to keep up with a growing spell-list is a pain). This way, if you want more spells, just download the PDF.
Book of the Archon will contain approximate 90 spells (the same number of spells found in the Sertorius Rule book). They are intended to create a campaign with a somewhat different feel from the core game. Therefore they are entirely optional, though spells can be brought piece by piece from Book of the Archon into a normal campaign of Sertorius. If things go well, we may release more spell books (for free) that each give the game a new atmosphere.