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.

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.