Richard Borcherds, a Fields medalist, has been putting out great math content on YouTube for the past year and a half (https://youtube.com/channel/UCIyDqfi_cbkp-RU20aBF-MQ). He even has lecture courses on a variety of undergraduate and graduate classes.
Fredrik Lundh once suggested the following set of rules for refactoring uses of lambda:
1. Write a lambda function.
2. Write a comment explaining what the heck that lambda does.
3. Study the comment for a while, and think of a name that captures the essence of the comment.
4. Convert the lambda to a def statement, using that name.
5. Remove the comment.
Alex Wright recently wrote a nice mathematical exposition on some of her work relating to Riemann surfaces, targeted at "a broad audience of non-experts."
A nice foundational textbook to start with is Michael Spivak's text "Calculus." This text is fairly conversational, it motivates its concepts well with many examples, and it will help you build a strong foundation in writing proofs and reasoning mathematically.
As others have said, it might be helpful to have a friend or two read with you so you can answer each other's questions. Also, make sure you do a ton of exercises!