Re-learning everything: Seeking general advice and book recommendations

In terms of generally re-learning topics like calculus or statistics, it's best to set aside a bit of time, like an hour or two, just practicing related problems. The easiest way to learn math concepts is by practicing them, and you could definitely use all the practice exercises on Khan Academy. You can find some here: https://www.khanacademy.org/math/calculus-1 and https://www.khanacademy.org/math/statistics-probability. 

Other than that, I understand if studying from a book is easier, so as a starter I would recommend Calculus for Dummies, 2nd edition by Mark Ryan. It is wonderfully written, really easy to understand, and walks through many practice problems. Barron's AP Calculus 14th edition is also excellent, though more fast-paced and complex. For statistics, Barron's AP Statistics 8th edition is a good resource. 

Hope that helps! If you have other questions, please do ask. 

Thank you!