bmitc

The concepts in SICP are timeless. There's nothing to be lost going through that book.

courses How to Code: Simple Data amd How to Code: Complex Data on edX that are based on HtDP.

The Spivak book is a good suggestion and might be a little difficult depending on their actual background.

I think I found out about this book from Hacker News. I haven't gotten to it yet, but I need to. Seems very unique.

If they know calculus, then Advanced Calculus: A Differential Forms Approach by Harold Edwards is a gem.

I highly recommend Head First C.

An Introduction to Manifolds by Loring Tu

The book Conceptual Mathematics is claimed to be aimed at high school students.

Another analysis suggestion is Creative Mathematics by H.S. Wall.

I really like Linear Algebra by Harold Edwards. He's an excellent writer and takes an algorithmic approach

the Scientific American Library series

The Elements of Integration and Lebesgue Measure by Robert G. Bartle.