The Art of Spellcasting

Age of Decadence's Vince D. Weller and Scars of War's Gareth Fouche have jointly penned an editorial that follows the evolution of spellcasting systems across quite a few classic PC role-playing games.
Games like Dungeon Master and Wizardry allowed players to select the power level they wanted to cast a spell at instead of automatically assigning a power level based on the mage's level. Mages could choose to channel more power into a spell than was safe, at the risk of failure and backlash. Wizardry 8 presented it simply and intuitively to the player as differently coloured circles. Green circles are safe for a given mage to cast, red is dangerous and everything in between involves some measure of risk.

The simplistic Diablo system showed a lot of promise, which was later abandoned in favor of a skill-based system. It fits the "magic is complex and can't be grasped right away just because you copied a spell into your spell book" aspect very well. Exploration has always been one of the strongest aspects of RPGs and the Diablo method is perfect for it as it provides the player with a reason to keep looking for more books/scrolls on the subject of a spell you already have and to be excited when another tome is found.

If done right, different books, scrolls, diaries, and notes can deal with different aspects of the same spell where one book can describe the casting methods, which increase the casting speed, while some arcane lab's research can help you understand how to feed more power into a spell. Think of Deus Ex weapon's upgrades system, where different upgrades could be applied many times, increasing accuracy, clip size, reducing recoil, reload speed, etc.

The "Spiderweb" (Avernum and Geneforge series) system does both "read more books on the subject" and "improve the spell with a trainer", which is a great combo. The former encourages exploration; the latter takes care of your money and gives you a pressing reason to earn more. When it comes to shaping (think summoning creatures that stick around), increasing the shaping skills grants you access to stronger creatures with different abilities.
Thanks, RPGCodex.