Fair point. Was going by the numbers of 3M USD seed round in 2012, and 17M in 2014. Without much major press and valuations being made public, it's kind of safe to assume that the 3M USD was burning out by the end of 2013. 14M from 2013 to 2015 would be 7M per year. Assuming rising costs from both running the business, 5M and 9M across two years seems plausible.

Given the number of employees in the company (26), 3 co founders, who form half of a 6 C-level team, it's likely that the team size increased along that curve as well (3M, 5M, 9M). This is entirely speculation at this point.

But after some further reading, it's possible that their business model may have just been not profitable. For each reader who read more than 10% of a book, the full retail price of the book is paid out to publishers. For people reading 3-4 books a month, they would be making a loss from those customers.

If you aren't the sort that reads 3-4 books a month, then you are also probably not the kind of person who spends a 10 USD subscription service to an 'all-you-can-read' ebook service. Therefore the larger base of customers there were probably loss makers. So assuming a growing customer base made up of loss makers it's entirely likely that they burnt out funds. Now that the running costs of a business in general were clarified in a reply above it actually makes sense that the money could have run out.