I honestly like Pielorinho's idea. I would go a bit further in my games as a house rule though. I would still give all players access to the abilities they retained out of, but I would use the out of practice reasoning in accessing a penalty to the attack rolls of that power. Anyone that uses an ability that they once had, but then retained out of it for another ability can use the old, out of practice power at a -4 or -5 penalty (play testing would be needed to determine the correct penalty).
I would further allow wizards one exception to the above rule. In a nod to their versatility, I would allow them to use the daily power that they did not prepare but could have prepared instead, at a negative -2 attack penalty. That way they can still use this ability if it is needed, but the penalty is not as high as normal since they have been practicing the ability but not as much as the ability they prepared.
Further, if you used the (alternate) power that you did not prepare, you could not use the power that you prepared and visa-versa. This should ensure that wizards do not get to use all their abilities in one day, just that any that they have learned and recorded are still available to them, if not at a significant cost.
In essence this means that a level one wizards could prepare sleep and have acid arrow as his back up. The place for sleep never comes us, so he would instead use acid arrow. By doing so, he makes his attack roll at a -2 penalty (he is practiced but not perfect with it as he did not spend the extra time familiarizing himself with the spell that is necessary to prepare it). When he makes use of that power, whether he hits or not, his daily power is used. He could not later on determine that sleep would be good, as he has already used his daily power on acid arrow.
When a power is replaced by another power, the wizards would take the standard -5 penalty to use the now replaced power, just like any other class.
As for books as treasures, I would be inclined to include them boss treasures.