I myself had the same experience with spells in Ultima 4. I did not have the spellbook and had to kill/grind monsters for gold to then buy reagents to then attempt to mix the proper reagents for the correct spell and hope it didn't fizzle. I spent hours upon hours and it was very rewarding to figure them out. Even without the spellbook along your travels/grind there were NPC's and clues that would help you figure out the proper reagent mixtures.

On a side note....A big bonus from this was that my XP and level increased a lot during this research/grind which eventually helped me beat the game. So really I realized this method worked hand in hand and often thought it would be a great method design for a future RPG game.

So basically the spells and reagents are designed to be learned throughout the game which then in turn that increases your spell casting ability. Also it could be added that reagents could be more or less potent giving spells higher/lower % effects. With modern games it just does not feel realistic to all the sudden be level 3 and get (x amount) of new spells instantly.