linjaaho writes "A group of Finnish mathematics researchers, teachers and students write an upper secondary mathematics textbook in a three-day booksprint. The event started on Friday 28th September at 9:00 (GMT+3) and the book will be (hopefully) ready on Sunday evening. The book is written in Finnish. The result — LaTeX source code and the PDF — is published with open CC-BY-license. As far as the authors know, this is the first time a course textbook is written in three-day hackathon. The hackathon approach has been used earlier mainly for coding open source software and writing manuals for open source software. The progress can be followed by visiting the repository at GitHub or the project Facebook page."