[Radek]

Nooo! Completely WRONG! The number of spells which you can learn at a particular spell level depends on your INT. If you run out of "free space" for the spells then you have to increase your INT. You can do it by means of potions. Therefore:
a) Collect spells which you cannot learn with your current INT.
b) Drink some kind of potion of genius.
c) Learn the spells.
Once you have learned the spells, your INT can decrease. The spells will not vanish. You will have more spells in your spellbook that you should have. That's all.