If users get to display ebooks they've written on their profiles, it could lend them a serious boost in credibility. But then again, not every book is equal. Either way, it's a promising expansion that could benefit Medium as a whole.
A publishing tunnel of sorts between Medium and Glose sounds interesting too, as long as it doesn't clutter what Medium stands for. It might, in time, compete with Amazon's established Kindle Direct Publishing. Give writers better rates and things might move quicker than one would expect.
I can also see this pay off handsomely in the Fiction and Poetry categories. They need all the love they can get!