Bankbook

Similar to the association account the savings book is used to deposit Credits.
Credits can be trasnfered from the savings book to the own association account or to other players.
Contraray, credits can't be transfered from the association account.
In conclusion, savings book credits are more valuable.

If credits are transfered to another player, they are booked on his savings book.
If credits are transfered to the association account they can not be transfered backward.

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