The reality is that the book will _always: be behind the bleeding edge—Rails just moves too fast. But I don’t see many serious deployed apps jumping to 3.0 in the immediate future—it just won’t be stable enough, both in terms of features and (possibly) bugs. So our decision is to document the latest in the 2.x tree. We’ll then investigate what we can do to provide ongoing coverage of 3.x, 4.x, 5.x and so on until !http://upload.wikimedia.org/math/b/e/4/be4c703ed73456618ed283b892c6715a.png!:http://en.wikipedia.org/wiki/Aleph_number.