I've just read Brian Kelly's post on formats for repositories, in which he talks about the EPUB format (see http://ukwebfocus.wordpress.com/2010/08/04/epub... for the post). He says:

'EPub is described in Wikipedia as “a free and open e-book standard by the International Digital Publishing Forum (IDPF)“. The article goes on to add that “EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device used by the reader of the EPUB-formatted book. The format is meant to function as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale.“

'In terms of the open standards used EPub consists of three specifications:

'Open Publication Structure (OPS) 2.0, contains the formatting of its content.
Open Packaging Format (OPF) 2.0, describes the structure of the .epub file in XML.
OEBPS Container Format (OCF) 1.0, collects all files as a ZIP archive.

'The articles states that “EPUB internally uses XHTML or DTBook (an XML standard provided by the DAISY Consortium) to represent the text and structure of the content document and a subset of CSS to provide layout and formatting. XML is used to create the document manifest, table of contents, and EPUB metadata. Finally, the files are bundled in a zip file as a packaging format.“'

It sounds much more "of the web" than PDF, and might be very interesting!