The two books in the most basic bundle cost $25 more on Kindle – it's a 5-star deal. (Update: we've been informed that there are way more stars than that.) Buy Now at Humble Bundle
Features
pay $1 to get Hua-bai Li's The Tai Chi in Star Formation and Abigail Pillitteri's The Universe Untangled
pay more than the average (currently $16) to unlock four extra books (six total)
pay $18 or more to get another 10 books (16 total)
