Please NOTE: I no longer recommend the use of PDFXML Inspector from Adobe and this presentation covers the use of InDesign version CS5 not CS5.5 which has significant improvements in regard to ePUB export..
You may also want to watch the instructional screencast on the same subject.
I have done something similar before but here is a fresh look at building a simple InDesign document, adding some paragraph and character styles and tags - and then building a relationship between them.
If we source a public domain text on the web, such as those from the Gutenberg project, we may well find that each line of thext has a fixed length. This is because the text was probably scanned in and then converted to real text through OCR software. The text is laid out exactly as it was in the original source. This facsimile is not appropriate for our needs.