Going from LaTeX to EPUB is a hard problem, and we believe it's not worth solving: if you're writing the type of book for which LaTeXis is the best choice, then your readers are certainly capable of reading a PDF, and they will probably prefer to do so, anyway, since they'll almost certainly be using a computer to read it.
For the most popular technical books on Leanpub, readers download the PDF over 90% of the time, and download the EPUB and MOBI versions less than 50% of the time.