The linked course pages contains a good synopsis, but Robert Harper's own blog post "What’s the big deal with HoTT?" [0] gives a wider view about its motivation and history.

Another two pages with worthwhile information on the HoTT book are [1] and [2].

[0] http://existentialtype.wordpress.com/2013/06/22/whats-the-bi...

[1] http://golem.ph.utexas.edu/category/2013/06/the_hott_book.ht...

[2] http://math.andrej.com/2013/06/20/the-hott-book/