If you are interested in software defined radio and interested in a way to learn its theory and applications, consider exploring the numerous instructional videos available on YouTube. You may not expect YouTube to carry much advanced mathematics, computer code, and theory useful in radio science, but it is there - and available for no cost. There is material pertinent for beginner through advanced levels.
Whether the topic is radio or computer science, mathematics, station building, or hardware reviews, it can be found somewhere on YouTube. Now, forsake those cute cat videos and never ending streams of political debates. Brew a coffee and take an hour or two to understand imaginary numbers. Spend a weekend learning how direct sampling radios are different from others based on QSD circuits.