Now that you've some knowledge on how to work with shaders I suggest you take a look at the following resources I gathered here for you. I'll try to have this list as much up to date as possible. If you think there is a link I should add let me know.
Forum
First of all be sure to join the Hidden People Forum, where we can discuss about the books and specific techniques you may be interested in!
Books
There are several books that I've found really important to read. These are not 2D-specific, but include broad topics that are essential to any game developer wanting to learn computer graphics.
Disclaimer: The links to the books below are affiliate links, by buying those books using these links you're helping with the production of the 2D Shader Developmenet series!
