A good way to learn more is to search for books about topics you're interested in e.g.; sqlite, zeromq, google testing libraries, opengl...

1. https://www.amazon.co.uk/Design-Patterns-Object-Oriented-Add...

2. https://www.oreilly.com/library/view/using-sqlite/9781449394...

3. https://www.glprogramming.com/red/

4. https://gameprogrammingpatterns.com/

5. https://zguide.zeromq.org/

Re: SQLIte, OReilly is coming out with a new book, Using SQLite, in August. It's not clear to me how much of an introduction it offers to SQL (versus SQLite specifics), but I'm guessing there's some introductory material in there as well.

http://oreilly.com/catalog/9780596521196/