Read through a curated collection of online documentation, where you’ll find information on everything from installing Backdrop CMS for the first time to creating your own layouts, themes, and modules.
Join the Backdrop Forum and exchange ideas with the rest of the community in the form of questions, answers, and discussions on any given topic.
If you're digging into the code and looking for some help, check out the API documentation and helpful developer tools available. Change records from Drupal are also available.
Two great books by Todd Tomlinson, author of “Pro Drupal 7 Development”