Uhm, reST is also easy to read and write, at least where Markdown is easy to read and write.

To me, they are largely the same thing, with slight syntax differences, with reST being more readable as it's clearer what any special syntax is about, whereas MD is more concise to type out — you need to learn the special syntax whatever way you slice it.

FWIW, reST was the default for Python "doctests": a wonderful feature that has fallen out of favour for some reason, but has the potential to be your real runnable documentation.

I wouldn't write a book in either, but that's because I am really high on great typesetting — I even had my own set of (Plain) TeX macros to do ToC generation, syntax highlighting, alternative math fonts, support for UTF-8 before eTeX and such when I did write papers and handed in coursework regularly.