They usually have to be downloaded onto your computer for use. This format contains enough information that it can be used for converting to other formats listed below, typesetting, processing by computer program, etc. I hope that most or all of the books will eventually be available in this format. When it is available, this is the source document from which other versions are derived. These files contain everything in a Word native file, and they look nice in browsers that support CSS stylesheets, especially if you have the fonts used in the document.
Text files are plain ascii, suitable for any text editor that can work with long files.
They do not retain all of the formatting and linking information of the other file formats. RTF is a word processor interchange format that can be read by Microsoft Word, Word Perfect, FrameMaker, and others. WinHelp is the format used by the windows help system. These files can be used on computers equipped with Microsoft Windows. These files retain all of the original formatting and can be viewed on the screen or printed at high resolution.