Ebook: The World According to Wavelets: The Story of a Mathematical Technique in the Making

I own the very first (French) edition of Mrs. Barbara's book.

It is nice reading, but not so fun as some would say.

It has some advanced calculus level math ( Fourier transform ) and treats mainly Haar wavelets.

As an introduction to the subject it is quite adequate, but if you need/want something more go elsewhere.