I recommend "Understanding Computation" from O'Reilly - it's pretty straightforward, teaches through examples and exercises, and uses Ruby, to build toy compilers from first principles.

It's not going to give you the mathematical basis that you'd get in a uni course, (it was actually 3 separate classes over 2 years at my uni), but it should help develop the practical _feel_ for when these things might help solve real engineering problems, which I think is what matters.