Foundations of Computer Science is a huge tome of an introduction to computer science covering data structures, logic, combinatorics, recursion, and more in its 700+ pages. Written by Al Aho and Jeff Ullman (already famous for 1986's "Dragon" Compilers book) in 1992, the book has now gone out of print, so they have the right to distribute it for free online.

When I say FoCS is an "introduction", however, I'm being liberal with the term. It's definitely a "serious" book and if you're not comfortable with reading academic material (and not everyone is), I think it might get too deep, too quickly, and with too few straightforward examples. Nonetheless, for free, it's now worth a try, whatever your level.