Bankbook

Similar to the bank account the bankbook is used to deposit Credits.
Credits can be transfered from the bankbook the own bank account or to other players.
Contrary, credits can't be transfered from the bank account to the bankbook. In conclusion, bankbook credits are more valuable.

If credits are transfered to another player, they are booked on his bankbook.
If credits are transfered to the bank account, they can not be transfered backward.

If you want to replenish your bankbook you can buy credits. How to do so and which payment options you have you can read here: Replenish credits.