There is only so much you can spend on books, I would suggest you register for an Algorithms/Data Structures course at a decent local university.

It could be the Sophomore level Data structures + Basic Algo course or a senior level Algorithms, wherever your comfort level is. Since you mostly do front-end development, this could bring serious depth in your computing skills.

For most CS programs, the data structures and basic algorithms course (typically at sophomore level) forms the backbone for most of the curriculum. If you didn't go through a CS program, this is the most important component that you might miss on.