Afaik no e-ink reader has MathML support yet, but most support SVG and embedded fonts.
The most future-proof way to write math formulas is still to use MathML (some HTML editors like Amaya and BlueGriffon feature tools for MathML) and make a SVG-only version of the book by using a XSLT processor and
pmml2svg to automatically convert all MathML markup of a HTML file to embedded SVG markup. You'll also need the STIXBeta fonts, so either it'll have to be embedded in the epub or installed on the reader (it's possible with PRS+ on Sony readers)