During this extraordinary COVID-19 time, Springer did an extraordinary thing that I never expected: They released a whole bucketload of books for free online! One of them caught my eye: "The Data Science Design Manual". Having browsed through the book PDF, I'm impressed by its coverage of the foundational topics that I think every data scientist should be equipped with: statistical inference, data wrangling, linear algebra, and machine learning. The author, Steven Skiena, also covers more in there.
Go grab the PDF while it's still free!