I'm looking around for the simplest way to transform a XML tree into EPUB. There are lots of tools claiming to accept quite a few input-formats. But most of them have less flexibility in defining user specified ways of transformations of the document tree. What I actually need is a programming language rather than a style sheet. So I've kept on trying to formulate the most straightforward way of transforming a given document tree into EPUB. I used to take advantage of Scheme with its representation of XML, which is called SXML. Now I prefer Haskell and its library HXT. Honestly, both are relatively less popular tools to tackle with XML data, but I think they are more powerful and sophisticated approach at least for programming-minded guys.