If you choose the first option to exchange Mastery Books, it will give you 2 choices: MB20 and MB30. Once you pick one of them, a list of the books you can trade in will appear.
After that, you're forced to close the dialog, even though it states you can go back to the first menu IIRC.
Okay, but then, the very first option, in which you had the 20/30 choices, simply disappears and you're stuck with the DW, Aran/WH, BaM options only.
Plus, not all books are eligible for the 20/30 trade in...
I guess one can simply make mules that will carry only MB20 OR MB30 and then try to trade those with the first option, I guess.
EDIT: okay, on one of my mules the first option didn't appear AT ALL. I have no idea of what's supposed to happen now.
Also, is there a list of answers for the 100% win event? All I got is this, but I dunno if it's complete:
http://www.basilmarket.com/forum/2758372
Sephie Wrote:500mil to class change.
Sad life... (being poor)
I guess you can use up a lot of char slots to make all Explorers you want to play as (just like I did...

). They did increase the limit to 30 after all