Since you mentioned you already have a strong applied math background, try Mathematics: Form and Function by Saunders MacLane for a broad portrait of mathematic

Since you mentioned you already have a strong applied math background, try Mathematics: Form and Function by Saunders MacLane for a broad portrait of mathematics that might get you excited to read more deeply into a given subtopic. Note that while this book surveys a broad swathe of mathematics, it depends on the reader already having a pretty high level of mathematical maturity. Other than that, if you want to go deeper into mathematics, just choose a topic and read on it. If your experience is mostly applied, consider something like topology which is often presented more formally than earlier math courses, or go back to the stuff you've already studied, but in a more pure context (for linear algebra, Axler's Linear Algebra Done Right and Halmos' Finite-Dimensional Vector Spaces; for calculus, Spivak as mentioned already and then an analysis book).