Simple, Secure, Scalable Web Development with Opa. The Opa book is, at first glance, a very comprehensive introduction to the Opa language and its motivation. What also excites: the installation under Linux is based only on basic packages, you don't first download half of the internet and install just because you want to use a language. (What is rather not so positive with Opa experiments: very long compile times - even the examples provided in the book, which are not exactly overwhelmingly large, quickly reach quite significant 15-20 seconds ...)