Read code from successful projects in languages you understand on github. Figure out why they're doing what they're doing. When you find something you don't understand, either search online for more context or ask another developer. You don't need to spend fifty hours a week reinventing the wheel, you just need to figure out why successful developers are successful. Books will close some of that gap, but nothing is as informative as understanding what and why something works.
Strongly agree with you that there are two ways to significantly and inexpensively improve programming skills.
1. read books written by experts, especially those with good reviews. These books are often categorized as software engineering or software development methodologies. If you find one of these books, then go to sites such as Amazon to see the relevant recommendations, and gradually you can find almost all of them.
2. read the recognized good source code. This is hard, but necessary.
Then it needs to be accompanied by practice: use what you have learned above consciously in practice, so that you can consolidate this knowledge and even you can build on it to gain more specific insights.
Low cost means: no need to create a special environment and no need to be overly dependent on others.
