FYI everyone, as posted on the early-dev board just moments ago:
http://forums.roguetemple.com/index.php?topic=4001.0this system has now been implemented.
The PC starts with a few different known spell-pieces (8 right now) in the future, you will be able to learn them from books instead of just knowing them.
The spell creation system uses a linear increase in time for more pieces added and a quickly ramping up mana cost for more pieces added.
The PC is limited to what kind of spells they can understand by how much mysticism they have.
Check it out if you have a chance!