I had to expand my browser to 1139px wide in order to avoid horizontal scrolling. This is bigger than a lot of people's max monitor resolution. It is certainly bigger than I prefer to keep my browser. I tend to keep mine around 950px.
The text on the homepage is very jarring. Preferably, use only one font size and color for any given paragraph. At most, use two.
The table for displaying info on books give a huge amount of space to information which seems to be relatively unimportant. Is the number of words (especially when it varies by only about +- 75) really that important? If you trimmed it down, you could display information on a lot more books on a single page. I hate paging through information, and most users won't even go past the first page (think about how you use Google or Amazon... how many pages are you typically willing to look through?). Some sort of categorization/tagging may also be in order.
The "get started" link on the homepage really needs to contain more information. If you want your users to start there, you should give them something more. I never start with signing up. I start with learning about the service.
The about page mentions that you can unlimited students in your account, but it's not at all clear what it means to have an account and what it means to have students in it. You are giving details, but I'm not clear on the basics, even after browsing around for 10 minutes. I've visited every page on your site and I still couldn't tell you what the day-to-day use of your site will be like.
I don't mean to criticize. I think you have a great idea, but I think right now, it comes off as very under-developed.