Hello, I would like to make a suggestion to improve the usefulness of adventure coins as well as to encourage players to complete the adventure book.

As you know to get the coins you must complete the achievements in the book; however, the achievements are unrepeatable, so the amount of coins is limited. Moreover, the rewards from coin exchange are not permanent, which means eventually you won't have enough coins to maintain them, and they aren't really worth your time investing into completing the achievements either.

So, my suggestion is to make the rewards permanent, but of course they are harder to get. Essentially, you have to complete all the achievements in the continent you are in and all the continents below in order to get enough coins for the rewards.
For example, I have calculated the sum of all achievements in continent 1: 775 points; including all the other achievements from general and other tabs, you might get up to 1000 points ~ 10 adventure coins.
Let's assume that there will be more achievements that you can complete on the upper continents (I haven't calculated but it can be done for proper measurement):
cont 2 - 15 coins, cont 3 - 20 coins, cont 4 - 30 coins, cont 5 - 40 coins, etc.
Now, the lowest unique you can get is Lv. 42, which is cont 3, my suggestion is to make it permanent at the cost of cont 1 + cont 2 + cont 3 = 45 coins per weapon.
Similarly, for Lv. 50, cont 4, you will need 75 coins for the unique weapons, as for the rare books, it should take 1/3 of the amount required for unique, which is 25 coins per book.

Most importantly, if you exchange for the unique and books from the lower continents, you would likely not to have enough coins for the upper continents; therefore, it is your choice to save the coins or to use them for the momentarily good-looking unique and books.

To make sure everyone won't be too OP, the change-effect books should be removed from the rewards so that they will only drop from Dimensional bosses.
Also, what if people create multiple characters since there is an option to exchange for account-bound coins? The answer is that the exchange will reduce the amount of coins by half, which is already hard to abuse, but extra insurance can be made by simply adding the requirement: a minimum level of 50 is needed to exchange for account-bound coins.

That's all, thank you for your patience and sorry for the broken English.