But didn't understand how that applied to my question. What should I do for development in Drupal?
Symfony 2, as in the full stack Symfony 2 has no bearing on Drupal 8 -- we do not use its bundles, we do not use the same routing as Symfony does (we use a botched up work on top called Symfony CMF which is made less dog slow by bastardizing my menu work on top of it but it's still undocumented, incomprehensible, does not yield to static code analysis tools and altogether is broken and slow). We do not use the same ORM (Doctrine). We are using some Symfony 2 components however there is very little visible of it to the "end" developer. For example, confusingly enough, sometimes you need to use events (what was hook_boot/hook_init in D7 is replaced by Symfony HTTP kernel events) whereas most often Drupal still uses its hooks. Confusing is the name of the game where Symfony 2 interacts with Drupal. It's an extremely poor fit and the (non)amount of the documentation of how this fit (does not) happen makes it even harder.
Edit: we do use the service container http://symfony.com/doc/current/book/service_container.html at least the static parts of it -- however dynamic service additions and alterations are done via Drupal specific means (service providers which attaches to the system via a special magic way which is neither events nor hooks. Welcome to Drupal 8.)
So: if you know Symfony 2 inside and out then a little of that will be usable in Drupal 8. If you don't then don't bother with Symfony 2 books or tutorials.
I "grew up" on Drupal so I was always procedural guy(Drupal is my main/only tool for making websites which is my job). At first I was very confused when I started to use Drupal 8. The OOP wasn't that hard since I knew OOP, but the new architecture was hard to get my head around. But after two weeks I started to feel like home again. This website and #drupal-contribute helped me a lot. I've read some information from Symfony documentation to understand controllers basically and no I'm working with D8 for 4 months and I feel like home again.
Things have changed but you'll learn the new ways very fast. You really don' have to learn Symfony AT ALL. I didn't.
One thing that made things MUCH easier for me was switching my IDE from NetBeans to PHPStorm because NetBeans didn't supported browsing inherited methods. After few months I cannot understand how was I able to use NB :) (though working with Git in NB is for me much better)
So to answer your question: you absolutely don't have to learn Symfony. To meake your life easier look at *.services.yml files in modules(that implements them) and also route callbacks in any modules *.routing.yml file to understand page callbacks(from hook_menu in D7).