SICP for CS and programming. That book (and its exercises) really instilled in me the thought patterns that drastically improved my programming skills and made me realize how important fundamental CS knowledge is to being a good programmer. What I learned from that book was: becoming comfortable with recursion, realizing the importance of abstractions, functional programming concepts and it gave me insights into how interpreters and the machine work.

I studied math in university and the book that really improved me mathematically was baby rudin. I really struggled with this the first time I worked through it and had to also fall back on Abott's analysis book, but after my first analysis course I worked through Rudin again and it just clicked. The exercises are really well chosen and the text is just so on point. Every sentence in the book is extremely carefully chosen and of fundamental importance to what Rudin wants to teach you.

It's really rare that you find textbooks that almost kind of have a certain magic to them, like you are directly taught by one of the greatest minds in that field. These two books definitely meet that criteria.