I've invested all the possible skill points into Magic of Miracles and Wisdom, so it's time to start pumping the second school of magic. I am tempted to invest into one of the forbidden books, as it just sounds so cool, but I've read the success rate will suffer if my alignment isn't in accordance with the magic school. Is this true? How big is this effect, especially if I'm good and want to cast Pagan Magic? And, most importantly, what is a good secondary school of magic? Something that will get me useful spells in the long run, not something that will lose potential as the time goes...

This sounds like a tough question to me because I found useful spells in most magical schools. I would suggest you read the Book of Magic and find spells that sound interesting to you and could fit your gamestyle. From the Divine Magic, I was using Divine Mercy/Holy Ground (in combination with a genie, these two spells allow you to transport back from a town to a dungeon) and Raise Dead (to revive my companions). From Magic of True Faith, I was using Lightning (as my primary attack spell for most of the game). From Inquisitorial Magic, I was using Light (to see in dark areas), Sixth Sense (to identify items for free) and Open Wounds/Crucifixion (before acquiring Lightning). From Heretical Magic, I was using Decay (to open locked doors and chests). From Pagan Magic, I was using Shatter (again, to open locked doors and chests), Ball of Lightning (this one wrought havoc in open areas full of enemies) and Rain of Fire (I got my hands on this spell close to the end and it proved very powerful against bosses).

And yes, it should be true that the success rate of spells depends on your alignment. But I don't know how big the effect is.

That was a very nice, thorough response :) Thanks a lot, Kovi! It's really amazing how much effort you put into supporting the community!

In my opinion, Inquisitorial magic is THE best magic school, Open Wounds/Stoning/Incinerate are the best attack spells for the respective parts of the game, Light and Sixth sense are pretty much required, crucifixion with a littel save/load is a great boss killer spell and Stigma in nice for crowd control.