Originally Posted by
Matuse
First priority is any new spells of a level you just obtained, so you have something to fill the new red boxes with.
Second priority is any spell that doesn't have a vendor selling it (presuming you'd actually use the spell, of course...a non-PM would have very little use for death aura). Firewall, Dimension Door, (Lesser) Death Aura, any 9th level spell, etc.
Third priority would be the highest level spell that you could use that wouldn't fit into one of the other categories. Higher level spells cost a hell of a lot more to buy and scribe, you may as well grab them for free if you can.
Fourth would just be filling in empty spots for spells that you haven't gotten around to getting and haven't had any luck in looting.