Reading Spells from Spellbooks

What's the deal? Specifically, what do you do for the following cases, in your own spellbook or someone else's spellbook (is there a difference?):

• 1) You read a spell that you know how to cast.
• 2) You read a spell that you failed the learning roll for.
• 3) You read a spell that is of a higher level than you can cast.
• 4) You have maxed out your number of spells known for a given level. Can you cast a spell of that level that you don't know from a spellbook?

Case 1: The spell is erased from the spell book; the mage must make a check based on his "learn spell" percentage, and if he fails, he forgets the spell cast. His total # of possible spells for that level is reduced by one Cases 2 & 3 I do not believe are possible. However, what if you attempt to cast from a spell book you've never tried to learn? This may be possible, but you would then forget a random spell of that level. Does this match previous game sessions? Discuss.

Kerry: While this does not match previous sessions, it seems fair and reasonable. However, note that folks (i.e. Fidimor) have often cast higher level spells out of books. It may be that those were merely bound scrolls, however.