Drupal at Your Fingertips is a free living Modern Drupal reference book created to help developers quickly find code they need. It is a community effort including other developers' input. Please share it with your network.

Some of the chapters include:

• Actions - Use the Drupal core actions module to control Drupal.
• Batch and Queue - Digging into the Batch API and Queue subsystem to take on large tasks.
• Blocks - Creating custom blocks, block types, block plugins, block forms, block visibility, block access, block regions, block templates, and block preprocess functions.
• Caching - Understanding the cache system, cache tags, cache contexts, cache max-age, cache bins, cache backends, cache invalidation, cache rebuild, cache clear, cache warm, and cache performance.
• Composer - Using Composer to manage dependencies and patches.
• Configuration - Managing configuration files, configuration entities, configuration forms, configuration schema, configuration split, configuration import, configuration export, and configuration override.
• Cron - Using hook_cron, sample crontab files, how to stop cron.
• Debugging - Using debugging tools, debugging methods, debugging variables, debugging queries, debugging routes, debugging hooks, debugging events, debugging tests, and debugging performance.
• Drush - Installing and using Drush, generating code, scaffolding modules, creating entities, updating database, debugging site, and running tests.
• Forms - Creating and altering forms, form elements, form validation, form submission, form redirection, form states, form storage, form AJAX, form theming, and form API.
• Hooks - Understanding the hook system, implementing hooks, altering hooks, invoking hooks, creating custom hooks, and debugging hooks.
• Menus - Creating and altering menus, menu links, menu trees, menu blocks, menu templates, menu preprocess functions, and menu API.
• Modules - Creating and altering modules, module structure, module dependencies, module installation, module uninstallation, module updates, module hooks, module events, and module testing.
• Routes and controllers - Creating and altering routes, route parameters, route requirements, route options, route controllers, route access, route responses, and route API.
• Services and Dependency injection - Understanding the service container, creating and altering services, service definitions, service arguments, service tags, service injection, service discovery, and service API.
• Testing - Writing and running PHPUnit and Drupal Test Traits tests.
• Twig - Using and extending Twig, Twig syntax, Twig filters, Twig functions, Twig variables, Twig templates, Twig debugging, Twig caching, and Twig API.
• Update - Upgrading and patching Drupal and contrib modules.

Note. you can buy a pdf version from leanpub.