Because of the complexity of tomes, players have developed various safety measures and rules regarding use.
When to use a tome: Generally, on a full server you will want to use a tome when there are fewer than seven players alive. Before using a tome, make sure the player who brought the tome (known as the “tomebringer”) has died first so that another tome may be brought in to replace the one being used. If there are multiple tomebringers, it is advised that all of them die before a tome is used so that one tome can give rise to multiple tomes. If they have already brought in two tomes, you dont have to ask them to die however.
It is generally considered good practice to tome dead players in before a boss is killed. However consider how many tomes you have left, and how many tomes are yet to enter the game before using them. You might not want to tome for Thor for example if there is only one tome left and there are no new tomes entering the game. Generally you want to clear out the spawn area(s) and get the boss as much under control as possible before toming in these situations, however you can tome if the boss is about to die if the players being resurrected are aware that they are about to respawn.