Are you asking about Leanpub? For this book I created the whole book using LaTeX and uploaded a finished PDF.

A good alternative is to write the book in MarkDown (Leanpub have a set of recommendations for getting good results). That works well for most books but for maths heavy books you can't really beat LaTeX.

I've never used their online editor. I always prefer to keep my work on my own machine as I have my own backup and change control that I use for everything I do.