On rerolling:
You can ONLY reroll a character that is level 1. So yes, it can have 900xp, but once it hits level 2 you no longer can use the book on it.
Furthermore, the moment you use a book, it gets bound to that account and you no longer can use the book on any other characters.
On to duallogging:
To duallog, you need a program called "Sandboxie", which is a free program which is easily found by googling. Once you installed, you can easily duallog by running a second fonline.exe sandboxed.
However, duallogging comes with some ruling attached to it. duallogging in pve situations is allowed, you can mine, kills npc's and do quests with two accounts no problem. However, it is forbidden to use duallogging to your advantage in pvp. This means that you cannot jump in an encounter with two accounts at once.
If a pker happens to jump in an encounter while you are dual logging in it, just leave one account idle (or run with it?) and fight with the other. If you fight with both accounts you are risking a permanent ban, as duallogging pretty much has a zero tolerance (Take a look at the banhammer thread in the pvp section).
On quickswitching:
Since we are touching pvp rules, i'll go over quickswitching aswell. It's generally bannable to instantly relog to a different character and jumping in the fight again after death. I recommend you to follow the rule of thumb where you can NEVER see your own corpse in a fight or encounter. So if you happen to have a character ready to fight, be sure to wait a few minutes (5 minutes is 100% safe) to jump in.