[Radek]

Thanks. Yes, I have a PC, therefore shift-left_click. Good.

As far as the casting chance is concerned, I have seen the following formula:

chance = (LUC/8 + WIL/4 + skill*2.5)*(0.6 + 0.4*fatigue/max_fatigue)

Notes:
1. Naturally, if the formula gives more than 100 then the chance is 100%
2. Perhaps, the WIL should be replaced by the governing attribute. It need not be always WIL.
3. The formula is a bit strange. It states that your chance to cast is 100% once you have the corresponding skill at 40 (with 100% fatigue points), no matter how expensive the spell is. On the other hand
a) yes, the chance goes down if you are fatigued
b) yes, the chance depends on LUC
c) at least for low level spells, yes, skill = 40 makes the chance 100%

Why to update the spellbook?
All depends on the behavior of the spells in Morrowind. In Arena and Daggerfall, the spell effect depended on your level. Therefore, you used a different set of spells at low levels, at medium levels and at high levels (different "base" and "increment" values). Moreover, the casting cost of a spell depended on your skill level in Daggerfall (and on your character level in Arena). Therefore, you used different spells at low skill levels (cheap spells) and at high skill levels (expensive spells).
I don't know how the spells behave in Morrowind. I would like to know... Only if the spell effect and the casting cost do not change and remains the same no matter what your level, skill or attributes are, it will be no use to update the spells in your "spellbook".