In 2011 SIAM launched an institutional e-book program, which makes SIAM books available by chapter in PDF form for readers at subscribing institutions. As of late 2012, SIAM books are now available for individual e-book purchase from Google Play, for use on tablets, smartphones, e-readers, or computers (but not Kindles). Unlike in the institutional program, these e-books are subject to full digital rights management (DRM), which means users cannot copy them or print from them and only the Google account holder has access to the book.
I’ve used the Preview facility to look at a few books on Google Play. My own SIAM books, such as Functions of Matrices (2008), are shown as “scanned pages” and appear to have been scanned from the hard copy; zooming in is supported.
By comparison, the Princeton Companion to Mathematics can be viewed as “scanned pages” or “flowing text” (ePub format). In the latter, which reformats as you zoom in and seems to be the default, the mathematics renders poorly; this is a shame given the impeccable LaTeX typesetting of the original book.
Is there a good solution yet for how to render mathematics in e-books?
Nick Higham 
ePub is, basically, a ZIP file containing a bunch of HTML files with some extras (CSS, images, meta info,…). Same as Open Document Format, it can (usually or always, I’m not sure) be opened by ZIP programs.
This means that the only question here is what the e-book readers can handle. The format itself is capable of carrying all the needed data, because the mathematics presentation on the web is a solved problem. For a while now, MathJax is the most popular solution and it does its job more than satisfactory. Before MathJax, we had jsMath, which was also a fine solution.
The problems with these solutions are of a practical nature:
1. E-readers’ support for JavaScript, in which both MathJax and jsMath are written, is non-existent or, at best, very limited.
2. One can hardly expect each reader to provide all the fonts that are needed, so each book should carry its own. MathJax install on my servers takes up 19MB, which is not much in terms of servers, but is quite a lot in terms of single e-books.
3. There is probably not much interest for this. Most people will not read mathematics on e-book readers, as the very limited screen space can easily make it an unpleasant experience.
I expect PDF (or a similar fixed file format) to remain the choice here, which is not very pleasant on e-book readers, due to the constant need for scrolling if zoomed in.
Maybe a PDF (again, or a similar file format), formatted exclusively for e-book readers (i.e., with the page size set to A6 or something similarly small, to avoid the need for scrolling on a single page), would provide decent results, but that would require quite an effort on the behalf of publishers which I don’t expect them to make.