yeah, modern gave a Int books while Ancient box gave a Advance book, so most of the time you can get Advance book that cost a lot in the ancient.. your just recycling the books if it's not usable then exchange it again to goncourt to increase the box that you'll be receiving
Selling ancient boxes i think is a more stable income but that works too if u get war cry. other adv books are really cheap
the free leagues specially when you always win is a great source of income. adv exp books also are very useful specially when you have lowbie mercs. copper coins, silver coins...woooosh... really good money fruits.