No, Unsealed Spellbook isn't OP. It is a good rune, but not OP.
Before the nerfs, when Unsealed Spellbook gave 25% cooldown to summoner spells it was used by a lot of different champions, even supports.
Now it only gives 15% cooldown on summoner spells. Unsealed Spellbook is still a good rune and it is still a viable choice.
At the moment it is mostly taken by Ryze and sometimes Vladimir, that's mostly it.
But no need to think about it too much right now, because Unsealed Spellbook is in for a rework. So you can expect it to be changed on patch 8.9 or maybe 8.10.
is it rue that if you change your first summoner for example...if you have ignite and flash...once you have change your ignite into exhaust for example...the cooldown for the used summoner spell id shorter than to its original cooldown?
If the cooldown of the exchanged summoner spell is shorter than the first one, yes.
For example: if you have flash on cooldown and you change it to ghost, the cooldown will be calculated for the remaining time on ghost. Which is shorter.
But if you would change ghost to flash, the remaining cooldown will be higher.
Before the nerfs, when Unsealed Spellbook gave 25% cooldown to summoner spells it was used by a lot of different champions, even supports.
Now it only gives 15% cooldown on summoner spells. Unsealed Spellbook is still a good rune and it is still a viable choice.
At the moment it is mostly taken by Ryze and sometimes Vladimir, that's mostly it.
But no need to think about it too much right now, because Unsealed Spellbook is in for a rework. So you can expect it to be changed on patch 8.9 or maybe 8.10.
For example: if you have flash on cooldown and you change it to ghost, the cooldown will be calculated for the remaining time on ghost. Which is shorter.
But if you would change ghost to flash, the remaining cooldown will be higher.