Free/open math textbook written in three days

Here is a free/open upper secondary mathematics textbook written in a single daythree days by a group of Finnish math teachers, working together in a "booksprint." Related news: California's passed a bill establishing 50 "open source" (CC-BY) textbooks for core lower-division college courses (though, as a poster on Slashdot notes, this still has to be funded in the California budget, which is a place where many good ideas go to die). (via Hacker News)