I think there's a lot of fun to be had with electronics when you don't use a microcontroller. Breadboarding, analog components, maybe some digital logic circuitry... And if you use a microcontroller, either not putting a linux distro on it or getting one that is incapable of running linux... For finding structured sources, you can do pretty well by just picking a college, finding their computer engineering program, some classes that look interesting, and see what books they require then buy them or see if a library has them. https://www.amazon.com/Digital-Fundamentals-10th-Thomas-Floy... or https://www.amazon.com/Digital-Electronics-Practical-Approac... are fairly good for instance but there are many others and some more appropriate for whatever your real study goals are.
It's really important to have a specific goal in mind regardless of whether you prefer to learn how to solve it with structured or unstructured materials, and to have an initial idea of what level of detail you want to go in order to accomplish the goal. The lower level you want to learn the less up-to-date your resources have to be, our understanding of electricity hasn't fundamentally changed since Maxwell's equations. :) Something like "I want to turn on a smart light of this type when there is motion in this area, and turn it off after 5 minutes" is a nice goal. And you can accomplish it at many different levels of hackery, and learn very deeply if you want about each component of your solution (for instance there's the entire field of RF if you want to know how the antenna works).
It's really important to have a specific goal in mind regardless of whether you prefer to learn how to solve it with structured or unstructured materials, and to have an initial idea of what level of detail you want to go in order to accomplish the goal. The lower level you want to learn the less up-to-date your resources have to be, our understanding of electricity hasn't fundamentally changed since Maxwell's equations. :) Something like "I want to turn on a smart light of this type when there is motion in this area, and turn it off after 5 minutes" is a nice goal. And you can accomplish it at many different levels of hackery, and learn very deeply if you want about each component of your solution (for instance there's the entire field of RF if you want to know how the antenna works).