Books
Modern PHP
The best PHP time saver book ever about the latest features of the language (PHP 5.5+), good practices and deployment. A must have for Drupal 8.
We talk here about OOP, namespaces, traits, generators, closure, PSR, components, optimization, deployment (Capistrano), profiling (XDebug, XHProf), unit tests (PHPUnit, Travis CI), dev environment (Vagrant), ...