Ask HN: What book to read to get a footing in CS theory?

A friend of mine has just taught herself the basics of JavaScript then done a code boot camp. She's pretty comfortable writing code now, but hasn't had a chance to get to grips with stuff like complexity analysis yet.

It seems to me like she's a member of a large and growing target audience for a book that gets you started with all the stuff you would learn in a computer science degree, with the assumption that you already know how to express an algorithm as code.

Can anyone recommend such a book?