Reddit mentions of The Art of Designing Embedded Systems

It's impossible to suggest a reference without knowing what your level of education is. Learning and understanding the details of embedded systems design basically sums up the second half of my four-year EE degree, so a good place to start is find out every textbook a university assigns for their EE degree w/ embedded focus and going from there.

"Art of Embedded Systems" is what I used but it's only the start.

e: I should note that book tends to stick to the software side of things. If you want to do physical hardware design that's a whole 'nother ball of wax.

Okay, you're definitely at the beginning. I'll clarify a few things and then recommend some resources.

1. Places to buy components: Depending on where you live in the world, the large component suppliers are almost always the way to go, with smaller suppliers like Adafruit/Sparkfun if you need development boards or specialised things. I buy almost exclusively from Digikey -- they have $8 flat shipping to Canada, which typically arrives the next day, with no customs fees. They have some sort of agreement in place where they cover these costs. This *always* saves money over going to my local stores where the prices are inflated. It's crazy how cheap some things are. If I need a few 2.2K 1206 resistors for a project, I just buy a reel of 1000 because they are so cheap.
   
2. "Steer a joystick with an app" Do you mean connect motors to it and have them move the joystick for you? You're going to want some sort of microcontroller platform, along with a motor controller and way to communicate with a smartphone app. You mention you know C++ so it will be easy to switch to C. This is both true and false. Programming for microcontrollers is not the same as programming for computers. You are much closer to the hardware, typically manipulating many registers directly instead of abstracting it away. Each microcontroller vendor has their own tools and compilers, although *some* do support GCC or alternatives. You mentioned PIC, which is a line of microcontrollers by a large company called Microchip. There are 8-bit, 16-bit, and 32-bit PICs, all at different price points and with hugely differing capabilities. Selecting the microcontroller for a project can be half the battle sometimes. Or, like me, you can just go with whatever you have on hand (which is usually MSP430s or PIC32MX's)
   
3. A lot of people will recommend the book The Art of Electronics. It's decent, but it's not for everyone. Some really like the conversational style, others don't. Many people who want to get into microcontroller programming and embedded development want to skip over the fundamentals and just get something working. For those, I point them to Arduino and let them on their merry way. However, if you actually want to learn something, I highly recommend buying an actual microcontroller development board, learning the fundamentals about electrical circuits, and programming in actual C with actual IDEs.
   
4. As far as resources go, again it depends on your actual goal. Whenever I want to learn a new tool (like a PCB layout software, or a new IDE) I always start with a simple project. Having an end point to reach will keep you motivated when things seem complicated. Your controlling a joystick with motors is a great starting point. I would buy a development board, Microchip PICs are popular, as are ST32s, and MSP430. It doesn't really matter that much in the long run. Just don't tie yourself too hard to one brand. Then pick up some stepper motors, and a stepper motor control board (grab one from Sparkfun/Adafruit, etc). Get yourself a breadboard, and some breadboard jumpers, a cheap power supply (there are tons available now for cheap that are pretty decent), and then jump in head first!
   
5. I highly recommend the book Making Embedded Systems by Elecia White, once you've covered the basics. It's a great way to learn more about how professionals actually design things. For the basics, you can watch *EARLY* EEVBlog videos (anything past around video 600/650 he gets progressively more annoying and set in his ways, another topic entirely, but the early stuff is decent). I'd also recommend picking up your choice of books about the fundamentals -- Electronics for Dummies, the aforementioned Art of Electronics, Making Embedded Systems, The Art of Designing Embedded Systems, and even stuff like Design Patterns for Embedded Systems in C. Again, it all depends on what your goal is. If you want to do embedded design, then you'll need to focus on that. If you're more into analog circuits, then maybe check out The Art and Science of Analog Circuit Design. Either way, grounding yourself in the fundamentals will help a LOT later on. It will make reading schematics way easier.
   
   I feel like I've gone off on a few tangents, but just ask for clarification if you want. I'd be happy to point you towards other resources.