Librie ebooks come in Sony's proprietary BBeB ("BroadBand eBook") format.
There are two formats:
BBeB book format
The BBeB book format comes in two filetype variants:
BBeB dictionary format
The BBeB dictionary format has only one filetype:
- .msd (BBeB Dictionary)
There's a whitepaper  (PDF) by Sony outlining the format.
LRF files are the most interesting format if you want to read your own contents on the Librie as they are neither signed nor encrypted.
You can create your own LRF files from plain text files and UTF-16 text files using the makelrf program available from http://groups.yahoo.com/group/librie/files/ContentGeneration/
Note: It'd be nice to have makelrf hosted as a webservice. Post an xml file and get the ebook back ;-)
The easiest way to create books is to use the "Printer Librie" utility and to print from any application under Windows. Unfortunately you cannot search or zoom books created by it.
The LRF file format
The LrfFormat is being reverse engineered for interoperability.