Those three file types made up the BroadBand eBook (BBeB) which was discontinued in 2010 when Sony began selling their ebooks in the EPUB format (which is more versatile and can be used on most devices with eReader capabilities). This proprietary file type is now obsolete, as well as the related LRS and LRX files.
Sony brand eBook eReaders (such as Sony Librie and Sony PRS-500 readers) utilize the LRF file extension for their documents and texts.
